It's spoofed. The email header should indicate that it is not sent from Electrum.org but via something else as it will almost always go through a few hops. Most email services will filter them out. It's entirely possible that the domain is just another that looks similar.

Does the key start with a K or L? If it is, you will want to follow the steps above to update your Electrum and open the wallet using it. It doesn't have a .dat extension, it is just a file.

If you only have the key, create a new wallet and Select the following: Import Bitcoin Address or Private keys and key your private key (K or L) into the box.

It's not. The gist of CPFP is to make another transaction with a higher than proportionate fee to subsidize the "parent" transaction so that the total fee rates for both transaction would be reasonable. It is definitely not bounded by wallet software but doing it in the wallet which you sent from could be simpler.

When importing BIP39 seed phrases, there are specific configuration for the addresses to be generated and can differ greatly between the wallets. IIRC, MyCelium uses BIP44[1] as the standard to generate the addresses. This is transparent to you and should no matter as long as you can do a CPFP within the wallet.
CPFP requires a high fee rate to compensate for the low fees of the parent transaction so that might be where your issue is. In this case, CPFP is not viable either unless you are able to send more funds into the wallet and spend both the change and the Bitcoins added. I don't think this is particularly feasible either.

You can't really tell if it rebroadcasts or someone else rebroadcasts it for you until it reaches the mempool expiry of the nodes. I would advice you to continually check the Blockexplorers and once you get an error from searching the transaction ID, make a new transaction.

Yes. CPFP works as the first transaction has to be confirmed for the miner to collect the fees from both of the transaction.

Unconfirmed transactions are still spendable, it is just that it cannot be confirmed until the transaction from which it is spending from gets a confirmation.

[1] https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki

CPFP works best if you are the receiver as it doesn't require another replacement transaction to be made or if you didn't select RBF(Replace By Fee) during the creation of the transaction. You can absolutely do a CPFP using the wallet to give miners an incentive to confirm both of your transactions. I'm not sure if Mycelium rebroadcasts your transaction, but it would likely take up to two weeks for majority of the network to drop your transaction and I would think that is way too long and possibly get you a confirmation sooner.

Installing Bitcoin Core and zapping your transaction is absolutely not a solution for your current situation. Rebroadcasting the transaction to keep it in the mempool is not helpful if your intention is to make another replacement transaction but it helps to keep it in the mempool and getting it confirmed once the fee gets lower.

Either:
1. Wait for the fees to drop which should hopefully be within the coming weekend.
2. Do a CPFP using the change that you currently have access to at the caveat of paying more fees.

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There's no double spend with your transaction, not relevant to your issue.

Electrum doesn't use BIP39 so selecting that would be incorrect in the first place and would have yield checksum:fail.

If you key your seed, it should show something like this:

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The only reason why it wouldn't let you continue is probably because either it is not an Electrum seed or that there is some error in the seed phrase itself.

No, it is a phishing attempt. Electrum does not and should not have your email address (other than the 2FA wallet, which AFAIK goes to TrustedCoin and not Electrum so it wouldn't matter). It also doesn't notify the users of any updates other than in the interface itself.

The wallet should detect that it is a 2FA seed when you try to restore it as a normal wallet. There should be something similar to Seed Type:2FA in the space beside the options. Don't select BIP39 as the seed type.

There should not be a case where Electrum fails to detect it as a 2FA seed in the first place.

Might be never. Blockchain.com can very well be rebroadcasting the transaction from time to time and it might never get dropped out of the mempool. Not sure what their wallet behaviour is but given that it has been 16 days, I suspect that is the case.

The transactions tends to drop out of the node's mempool after two weeks so if every node received the transaction at the same time, it would have been dropped. Problem here is that it could be kept within the node's mempool if someone broadcasts the transaction again to ensure that it never drops out of the mempool.

There's no such thing as being forgotten by the network. The miners simply chooses the transaction that they see fit; it could very well still be mined by a miner after you see it disappear from the wallet.

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If you're the receiver, I can't see how you can't spend the unconfirmed transaction to do a child pay for parent as mentioned by DireWolfM14. The caveat is that this can take a huge part of your payment and isn't fair for the cost to be borne by you.

They mentioned that they may or may not convert the Bitcoins into fiat in their filing so it's quite ambiguous and I would imagine that it depends largely on the market outlook.

That's a nice find OP, not sure why they opened that to the internet though.

If you want to do that, would it be better for you to create a raw transaction without any inputs and use fundrawtransaction to add the inputs instead? If you were to specify the fee rate as well as the deduction of the fees from the outputs, then it would be far easier than to calculate the fees manually. See this: https://bitcoincore.org/en/doc/0.18.0/rpc/rawtransactions/fundrawtransaction/.

It'll make more sense for companies to be making miners in the regions for which they're concentrated at. Bitmain entered the game fairly early and could afford to mass manufacture their ASICs at a lower price and a lower shipping cost since it's within China. Bitcoin ASICs or any mining ASICs are a very niche kind of chips and they're only useful for one thing and I presume the demand in US is actually not particularly high or sufficient enough. The profit margin can get razor thin for both the US miners as well as the companies.

Wasn't ButterflyLabs based in the US? And also some of the other mining companies?

I think paper wallets are not terrible as it is basically just another way to store your keys or seeds. The points as stated in the wiki are completely valid and are indeed downsides of paper wallets. Generating a paper wallet would most likely involve an offline computer so rather than doing that, I'll just make an Electrum wallet, export the master public key, write the seed and laminate it.

If you want to make it more indestructible, use some metal stamping tool to etch it into a block of metal.
Here lies another problem; that hasn't been any new commits for the past few years which means it doesn't have segwit support. Not exactly a big problem but it works fine if you want to generate legacy address.

Here: https://jlopp.github.io/bitcoin-transaction-size-calculator/

What wallet are you using? Most wallets should let you define your fee rates and give you the transaction size at the point of signing as well.

When typing the seed phrase into the restoration box, Electrum will indicate the seed type as old, which is basically the seed system used in the earlier version of Electrum. Which is what happened with OP.

It's not a BIP39 seed. It's a seed generated with Electrum's older seed format.

Electrum has a specific formatting byte at the start of the seed which identifies its version. If the wallet doesn't contain anything, then I'm afraid it's probably empty. I'm not aware of any non BIP39 seed phrase with 12 word seed. Blockchain uses 16 words AFAIK.

No. An input is roughly 68vbytes and 10 inputs would be only 680vbytes (0.68vKB). The limit for the transaction to be considered standard is 400,000 weight units or 100,000vBytes. What he meant was that splitting your (presumably large) consolidation into multiple of them would give the transaction a better chance to be fit into a block. Ie, Having a 10KB transaction when the miner can only fit 9KB left, the miner could possibly choose to include other transaction so the space is optimized.

I don't think there is any harm to have a larger transaction as long as it's under 100vKB and you have flagged it as opt-in RBF so it's easily replaceable if needed.

Yes. Miners can choose to not include it. There can be a whole lot of factors as to why miners would not want to include it; non-standard transactions, fees are too low, miner's preference, etc. No one can really tell you the odds of it happening; generally miners do not actively exclude specific transactions and non-standard transactions should not happen if your wallet is functional. I vaguely recall there's one mining pool that mines non-segwit transactions exclusively.

For it to be never included in the block signifies that all of the miners collectively do not want to mine it and it's almost always because it is invalid or non-standard.**

Depends. See above, it'll only be confirmed if it's included in the block in the longest chain.
Weeks.
Possible. How much fees did you pay? The minimum fees for a good propagation is 1sat/vbyte and can increase depending on the mempool capacity (which I assume isn't the case). Poor propagation is not all that common anymore, what wallet are you using?

*Non-standard transactions can still be included in a block but they require explicit participation from a miner that is able to mine a block.

Electrum's seed format has specific version numbers embedded and it will automatically choose the kind of address you'll generate, which is legacy in OP's case. You'll have to generate a new seed specifically for Segwit addresses if you want to use it.

Yes. The error shows when the entered password doesn't match the actual password of the wallet.dat. It is independent of the error that may have been present in the user's entered password.

Just restore the wallet on your computer with the 12 words seeds that you were given during your creation of your mobile Electrum wallet. Note that the fees would still be similar but you should be able to choose your own fees. You will want to create another wallet with the bech32 addresses (bc1) to pay lesser fees in the future. You can send the funds to that wallet when making your transaction.

* Fees appears to be fairly low right now and you can probably get away with paying ~13 sat/vbyte and get a confirmation within the next few hours.