"on all"? no.
any mining pool that starts censoring transactions will abandoned be immediately by the miners that connect to it. the more serious the threats gets the more new mining pools in different places of the world will start for miners to migrate to. the more serious it gets can also lead to a mining protocol change that bricks all their efforts.

i can't even find this pool among the list of bitcoin mining pools!
the smallest listed here (https://btc.com/stats/pool) has 0.22% of the total hashrate.

people aren't saying bitcoin is the only thing that can succeed, they are saying that others aren't trying to succeed. instead when you look at altcoins you can clearly see that their goal is to get the biggest pump and dumps and make the most amount of money from it.

the reason why everyone hates ETH so much is because of its numerous issues. for starters it is centralized and mutable. two fundamental characteristics of a successful project simply doesn't exist in this altcoin. the only innovation that it had was the idea of smart contracts that are more advanced that what bitcoin (the currency) allows. but that idea was executed very poorly on a buggy platform. and that's just the tip of the iceberg.

as for PoS that you mentioned here, it is a very poor algorithm for a cryptocurrency. it has lots of attack vectors and adds certain weaknesses to a cryptocurrency that it automatically guarantees its lack of security.

you can't sweep an address, and if you import it then it will be watch-only meaning you can't spend from it. you need private key to sweep or import while being able to spend from that address. as @jackg said you can do it using Electrum but know that you have to create a new wallet and in the creation window that appears you have to enter the key with its script type otherwise you can not import a key into an existing wallet with seed.

also be warned that sweeping is mostly about avoiding any possible vulnerability in the original wallet. otherwise if you import the key, it will still remain in the original wallet. if it were vulnerable (a web wallet, a wallet that had broken RNG, the original wallet gets hacked some day,...) you will lose your funds.

you also didn't mention what wallet you are using. depending on the type, it could be a variety of reasons. a custodial wallet or web wallets may have been bugged or they simply hadn't credited your account, in Electrum you may have clicked the wrong button and stored the tx locally instead of broadcasting it, in bitcoin core you may be out of sync/having network problems,...

it doesn't matter how it is labeled, as long as something that is designed to not have any value is being sold, that is the definition of scam.

bitcoin being the world currency has nothing to do with testnet-bitcoin being sold!

testnet is already stable.

testnet is already both easy and good to use for testers. i have never had any issues using it.
if you had any issues YOURSELF then please share your issue and lets discuss why YOU are facing an issue. if you are just concerned about the number of people who have been complaining lately, know that they want to get large sums of tbtc so that they can exchange it for some scam token that may have a value someday if it doesn't die. they are not testing anything!

any application has different layers. imagine if you wanted to test if the car's window button (roll up handle), you just focus on that button not fill the tank with gas and start the car, go in highway and test the button while driving!

in your example the decimal issue is a UI problem, it doesn't even get to backend. to test it, we focus on that part only. there is absolutely no need to create a transaction and broadcast it to test something as simple as this. all it takes is a good code, preferably following IoC principle then we write test cases that are testing the edge cases on the UI thread only. then the same tests on backend but this time it only accepts int-64 instead of decimal (decimal only exists in UI) and similar edge cases are tested there too. same with any other part.
then all these parts are joined together and work like a well oiled machine.

long story short, in my experience whenever a developer is complaining about hardness of a certain test, that's because their code is bad and needs a big refactor.

Actually 1 private key creates 1 public key (point) and that point can be represented in 3 different ways: compressed, uncompressed and hybrid. if you has each representation you will get a different hash hence a different address and all are valid but some are non-standard.

you areoverthinking it. different addresses are no more than different representation of pubkey scripts. as long as your wallet application is capable of performing hash on your public key and knows how to spend certain scripts (eg. P2WPKH script) you can spend from any of the "addresses" created from your private key.

it really depends on "why" of it. bitcoin is not a person to have a car accident and suddenly die
considering the fact that altcoins are copies of bitcoin in one way or another, 98% of them die right away because whatever thing that can cause bitcoin's death will be affecting them too. that 2% are the rare and mostly unpopular projects that are completely different from bitcoin otherwise shitcoins like BCH, XRP, ETH, USDT, BNB, LTC, ... are copies of bitcoin technology so it doesn't matter if their blockchain is different.

when you derive a key using KDFs (like the second part of my initial comment) or if the key was derived using a hardened path in BIP32 scheme then everything is irreversible and even if any child private keys were leaked the master key (or the seed) will never be revealed. it is simply impossible because the process which consists of mostly SHA512 hashes is not reversible, ever.

i would have mixed feelings. a little bit mad that they are scamming people, a little bit sad that idiots exist (both those who created the market and those participating  in it) and a lot glad because it would mean the days of Testnet3 are numbered and it signals move to TestNet4.

it would be even nicer if people who aren't competent in programming and are unfamiliar with bitcoin stopped creating applications, so they won't need any coins to test those applications with in first place.

a "billionaire" will never reveal where they are investing their money. they need to protect their privacy more than anybody else and why would they do it anyways?
a hedge fund manager on the other hand mostly advertises their own service and lives on commissions. if this dude were investing in bitcoin, he has been doing it for a long time not just now that price is up and the market is being hyped up. meaning when price was $3k. telling others about it means he wants others to invest in bitcoin possibly through his firm!

just being around for longer time and having a fake-big market cap doesn't mean an altcoin is established and definitely doesn't mean it is a good investment. both altcoins you named here are centralized and terrible investments in long term. they are both only good for pump and dumps.

considering that you are talking about "cryptocurrency" investment and not just bitcoin, talking about "investment" and "holding" is a bad idea because majority of altcoins are terrible investments since they have no long term potential and they have always been losing value in long term. that means holding them is a guaranteed loss. in altcoin cases selling when you see a dump is the wise thing to do.

there really is no fixed timeframe for these things because bitcoin price rises are greatly affected by a lot of different factors (most of which involve factors affecting the adoption). for example anything that can speed up the adoption such as countries adopting bitcoin as a currency (eg. Japan in 2017) greatly increases the adoption speed and with it the price reaches the next ATH a lot faster. on the other hand opposite things that can slow down adoption greatly decrease the speed and increase the time it takes to reach the next ATH. for example the recent pandemic panic sell or a economic crash/recession could slow it down.

my guess is that the previous ATH will be broken next year in first couple of months and within first quarter we will see new ATH around $35k to $40k and by the end of 2021 we will have the final ATH above $50k and possibly around $100k.

i believe the problem is the new dependency requirements of BIP38 (it needs implementation of scrypt and AES, although i think Electrum should already have AES). there was a PR that was closed because of this a couple of years ago.
there is also a low priority issue here: https://github.com/spesmilo/electrum/issues/3786

using one seed to generate everything (your bitcoin keys, PGP key, etc.) also adds an additional risk that if you lose or leak that seed you will lose everything.

you can just read the first 2 paragraphs on Wikipedia page on Web of Trust (https://en.wikipedia.org/wiki/Web_of_trust) to learn what it means. essentially it comes down to you NOT just open the wallet website and get the public key from there (Electrum or any other software) which would be the same place you downloaded the binaries from.
you can already check that the pubkey hash is already posted in multiple places and is 0x6694D8DE7BE8EE5631BED9502BD5824B7F9470E6 (eg. on electrum docs). you can also check the Electrum GitHub repository to get the same public key (https://github.com/spesmilo/electrum/blob/master/pubkeys/ThomasV.asc).
also if you have a friend whom you trust and have their PGP pubkey you can ask them to sign Electrum's pubkey with their key and send you the signature so that you can verify if you have the correct Electrum PGP pubkey.

it is not just sellers. it is buyers too who buy bitcoin from exchanges and pull it out to their cold storage. it is also a lot of shitcoin traders who dump their shitcoin and move the bitcoin to their bitcoin exchange to trade bitcoin instead while altcoins dump.

such a strange statement about bitcoin.org followed by recommendation of a closed source wallet!

any wallet that has not released their source code, can not be reviewed by anyone, is not reviewed by experts and doesn't have deterministic builds is considered shady and must be avoided. ownbit wallet fails on the first step since it is closed source. and worst of all they have pretty much a useless excuse for being closed source too: "we are not opening yet for preventing copy & paste from competitors".

i seriously doubt that either one of these old men in their 70s even know what a cryptocurrency is, or have any more knowledge other than maybe a footnote in a memo they were handed.

that is correct. without having the unencrypted wallet file itself or the master key of HD wallets, there is no way of linking different addresses together claiming they belong to the same wallet.

a wallet creates the signature from the private key (not the pair) and in a transaction you publish the signature + the public key, and both of these are used to "unlock" the transaction output that is being spent, or in simple terms spend bitcoin.

yes this last one is the best explanation of what a wallet is. although wallet could simply refer to the file or the medium used to store the key(s) like paper wallet or wallet.dat file that only contain the key(s).

there is no tool or proposal for doing something like that in bitcoin world if that's what you are asking. but the principles are the same. you have a seed, a key derivation function, and child keys derived from that seed. so all you have to do is to choose a BIP-32 derivation path (eg. m/3146786'/9012455'/0 random path to avoid using anything that may be in use) and derive a new bitcoin private key (ie. a 256-bit number). then use that as your PGP key assuming you want a 256-bit long PGP key (i believe it is safe if you choose ECC algorithm not RSA).

if the PGP key is not 256-bit (eg. 4096 bit RSA or 384 bit ECC) then you have to take a step back right before going into BIP32 and use a different key size in PBDKF2 when you derive the BIP-32 seed using the mnemonic, and use that derived key as the PGP private key.