Yes. As long as the zpub doesn't corresponds to the 2 seeds you've stated.
Technically. Since your Multisig is generated with the list of public keys from those xpubs. However, if you can somehow figure out the redeem script of the address that the coins are in, you can still spend them. It is unlikely that you would be able to do so since the whole point is to discourage address reuse.
Yes.
No standard for those.

No reason to dictate what he should do with his coins, to burn them or not. He is free to use the coins as and when he likes, the fact that it hasn't moved yet doesn't necessarily means that he intends for it to be burned regardless.

It depends. I've had terrible experiences with certain laptops when running them for extended period of time. I would rather not stress laptops (particularly their battery) by having them run 24/7, because they are not designed with that in mind. You are far more likely to have it fail on you than a brand-new Pi and even if it does then you only have to replace the USB charger.

The reason why Pi has passive cooling is because it doesn't really need active cooling system, perhaps save for a little extra heatsink on top of it. You are more likely to save more money going with a Pi, at least I've never really seen any computers that would be far cheaper than Pi (less than $50??) and isn't a headache to use. You're probably either going to find it grossly inadequate for Core or have a certain component fail on you after a while, that is the problem with buying old hardware, I've had quite a fair bit of experience in this regard.

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Anyways regarding the cost, if you need to be concerned about the cost of running a node then you should probably spend your money elsewhere. Not everyone has to run a node, especially if you can't really afford it financially.

There isn't. Running a Bitcoin node carries no monetary incentive nor is there a need for one. Whilst the network definitely needs full nodes to function, the importance of an average Joe running one can often be misconstrued. An bunch of centralized full nodes is no better than a far lesser number of full nodes but with vastly larger decentralization. There is little to no opportunity costs for most when running a full node, and it doesn't necessarily mean that it has to be turned on 24/7. The actual cost for it is far lower than the 'perceived' benefits to self and to the network.

I don't think dynamic IP is an issue here, if so then using DDNS would solve the issue as well. The local IP assignment can be changed to be static and it usually doesn't really change so the port forwarding sticks.

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For starters, 8333 is the wrong port. You are looking to access your RPC from outside your LAN, which requires you to portforward your RPC port, which should be 8332 by default. You don't need to accept incoming connections for your RPC usage.

RPC is by default only enabled within your local network. If you want to expose it over the internet, then you can port forward it. Fair warning though, be prepared for scripts hammering your RPC logins once you expose it. I would recommend you setting up a VPN instead.

You can specify your own RPC port, but it is usually 8332 by default. Port forward that on your router, then give your friend your IP and port.

RPCs should ideally not be exposed to the internet, nor should it be unsecured. The RPC within Bitcoin Core is not encrypted, so it would be a better to run a network tunneling, ie. VPN to your friend's computer.

What are you intending to do with your mnemonic? You are free to choose any derivation path you'd like as long as you are aware of the derivation path that you've chosen. Some wallets have specific derivation path support only, for which it would be better to choose one that is supported by your wallet from the onstart.

You should, as far as possible be spending to an address that is un-used at the lowest index and do it sequentially.

The signature of a firmware means nothing if you don't trust Trezor because you're not independently verifying the code. The purpose of a signature is merely to verify that it is coming from a source that you're expecting. Assuming that your Trezor is not compromised (ie. not tampered with during shipping), then it will only accept signed firmware from the manufacturer.

It might not be so economical, if the fees are low. Each of the outputs contribute significantly to the fees and it might not be worth it to collect the fees individually, rather than have the merchant pay you during the settlement.

BIP70 allows the merchant to specify multiple output as part of the payment request. You can perhaps try to explore that further. I'm not sure about the multiple output compatibility with the wallets though, IIRC it is allowed as per the proposal.

Trezor uses a set of specific derivation paths. So long as your wallet supports BIP39 seeds and allows you to specify your own derivation path according to their standard; https://wiki.trezor.io/Standard_derivation_paths. Then you can import it into that wallet. Electrum allows their user to do so, not so sure about Blue wallet.

Your seeds should always be generated on the hardware wallet and not exposed otherwise. Importing seeds that has been generated elsewhere basically gives you the security of the weakest link (ie. the device for which you've generated it in).

To answer your question, you can't. Electrum seeds are not compliant with BIP39 standards and thus cannot be imported into Trezor.

The first few characters of the address. In the earlier days, putting the first few characters of the address into a blockexplorer will yield the intended address. It isn't really that applicable right now.

Just to be clear, I don't think the issue here is about the trust issues on the manufacturer. You will never ever know what does the private key correspond to unless you peel it, which is quite unlikely for most people to do. It only provides a shortened representation of the actual address, for which you will always need a list of the address.

I forgot if UPnP is turned on by default, and I'm assuming the torrent client uses it to open the port. Can you go to Settings>Options>Network and check Map port using UPnP?

Most exchanges has ToS that either allows them to restrict your account or bar you from using the platform completely in the case of having deposits being associated with mixers or anything that goes against their ToS. Mixers have a distinct pattern and behavior that makes it trivial for exchanges to identify them, but it still doesn't break your privacy. If you are using a mixer and a centralized exchange, then you should probably reconsider your priorities; centralized exchanges will do anything to track their customers. Using a mixer merely makes their job slightly more difficult.

If you're running through a VPN, then you should check with your VPN. Are there any steps to enable portforwarding using your VPN? If canyouseeme.org shows false, then the problem is with your portforwarding and not Core.

Yup. The setting is only disabled if you're running a proxy or through Tor. I was asking that on the premise that you might have specified that, given that it is turned on by default.
This is correct. It is good to be able to have inbound connection, for the network but it doesn't necessarily mean that you're not contributing to the network. Even if you do enable inbound connection, then the primary benefits should fall on you and the overall benefits on the network is secondary.

The bugs does not affect the network permanently. In the case of any inflation bug happening, then part of the community will rightfully split the chain to fix the inflation bug while letting the other chain (presumably) die off. Obviously, there is a chance that the network just survives alongside Bitcoin, but it doesn't threaten the security of the network.

There are numerous attack vectors within any decentralized network, many of whom are theoretical and hard to execute. With every vulnerability, we attempt to mitigate it by minimizing or completely eliminating the impact of it. Those attacks are discussed on the mailing list, if you want to know more about them, search it up using the mailing list. All your concerns on specific vulnerabilities should be addressed.

When using firstbits, you're likely to get quite a few results and potentially being unable to differentiate them from each other, assuming that it remains funded. If you have the private key, then there is no issues as you're probably still going to recover the entire key and if you don't have it then there is no point finding the address anyways.

Firstbits is useful if you're for example, someone who uses the addresses in a constraint space, where you have a limited number of characters to do so. Then it would be fine to differentiate them according to the first bits provided that you can maintain that there will not be any collision within your own firstbits and maintain an index of it.

You're never spending multiple transactions, you are spending multiple transaction outputs. It is possible and easy to do so. Just reference all of them in the transaction that you want.
All of them. The transaction needs to reference all of their TXIDs and their corresponding indexes.

You select the inputs that you'd like to spend in the transaction. Get the TXID and the output index of it for the transaction, for which the output index is the order for which your output is in within that specific transaction.

Electrum obtains the unspent transaction output and displays it to the user in the GUI for them to choose and construct an unsigned transaction for it to be signed, that is the purpose of a watch-only wallet.