Sorry, it can't be done.
The fee is the difference between the input(s)' and output(s)' amount, not exactly indicated in the transaction.
So to edit the fee, you would need to change the output's amount which will invalidate the signature.
It's possible only for unsigned raw transaction.

BTW, mempool isn't too bad right now and 6sat/vB wont be dropped from nodes with the default settings.
Try re-broadcasting it and then perform CPFP (Child-Pays-For-Parent) to bump the overall fee rate.

You must be using a legacy wallet (non-descriptor).
The wallet where you execute the command should have the "disable_private_keys" argument set to "true" when it's created.
In the GUI, the check box with the same name "Disabled Private Keys" should be ticked.

In Descriptor wallets, it's not possible to import plain address descriptors to a non-watching-only wallet so this wouldn't have happened.

I don't know how it's possible with this requirement since bitcoind is a Bitcoin full node.

Yes, bitcoind will run independently, and you can communicate with it using RPC or other methods. (e.g.: bitcoin-cli)

Here's the developers guide for Bitcoin Core: https://developer.bitcoin.org/devguide/index.html

There's nothing special in the setup aside from creating an extra watching-only wallet on an online PC for transaction creation and balance monitoring.
You just have to create a wallet on an "Air-Gap" PC (WiKi about Air-Gap setup), export its "master public key" and import it to an online PC.

This is not a video guide but these pictures and instructions should be easy to follow;

On your Offline PC:

• Create a new wallet, start by typing your preferred wallet name, press "Next"
  
• Select the option "Standard wallet", press "Next", then in the next window select "Create a new seed".
  
• Your 12-word seed will show, copy this to a paper or other forms of physical backup (e.g. metal plates).
  After copying it, Press "Next" and Electrum will ask you to retype the seed to confirm if your backup is correct.
  Finish the rest of the steps.
  
• After creating the offline wallet, open the menu "Wallet->Information"
  
• This "Wallet Information" window will open.
  Find a way to copy the master public key; either by manually writing it on a paper,
  or copy-paste to a text file to be transferred via flash drive, or use the QR code icon that can be scanned in the online PC's camera during the watching-only wallet creation.
  

On your Online PC:

• Create a new wallet by following the same steps up to the selection of "Standard Wallet",
  but instead of creating a new seed, select "Use a master key" instead, then press "Next".
  
• Paste or Scan (via the camera icon) your master public key, and finish the rest of the steps.
  
  
  

To be sure that you've set-it up correctly, you can check both wallet's "Addresses" tab if they're identical.
(Open the menu: "View->Show Addresses" to enable the tab)

For imported address within your wallet., you can use listreceivedbyaddress but with the address arg to filter it.
Otherwise it'll list the whole wallet's receiving addresses' TXID list. (excluding change for HD wallet)

Example command:
(works with imported addresses via importaddress)

Based from your description of "long password w/ symbols", it sounds like it's in BASE64 format.
Does it looks similar to this? (or something longer):

IDK if it's an Armory backup since I haven't used the 2012 version.
Or it might be an individual private key that you've imported to Armory so it's worth trying to decrypt it and encode into WIF format.
It's hard to tell with the current info.

For reference, Armory's "Root Key" and "Chain Code" both look like random groups of four letters separated by whitespaces.

Have you added your Electrum wallet's master public key in your EPS' "config.ini" file?

You can get it from Electrum menu "Wallet->Information" and paste it in the config in this format:
(remove the "#" from the sample)

Then restart EPS.

Still Electrum seed if you're going to use Electrum.
The cons of using BIP39 seed is you'll have to take note of the script type and the extra steps when restoring them.
And also, there's a possible future compatibility issue since it's not recommended by Electrum itself. (check the "Warning" when restoring BIP39 seed)
The pro is of course, it's compatible with almost all non-custodial wallets.

With Electrum seed, you can directly restore them in Electrum without taking note of the script type.

Anyhow, both backup should still have a note of your selection of "m-of-n", number of required signatures and number of cosigners.

You remembered correctly, just use the command getblockchaininfo and look for "pruned" among the results.
It's either "true" or "false" which is self-explanatory.

You put the "bitcoin.conf" file in your selected bitcoin data directory in the GUI's welcome screen (first time launch),
if you're using the default data directory or bitcoind, then put in: https://en.bitcoin.it/wiki/Data_directory#Default_Location

Right, just the receiving address since there's no blockexplorer that accepts multiple Master Public Keys to get a MultiSig wallet's entire balance.

By saying "internet", do you mean via blockexplorer?
And by saying "balance", do you mean the entire wallet's balance?
If so, most of them do not accept multiple master public keys to check for transactions of your MultiSig Electrum wallet.
It'll only work on standard wallet's master public key.

Otherwise, just paste your MultiSig address and it'll search just like in SingleSig addresses.

Yes, even though it's labeled as "sat/Byte" the actual computation done is actually "sat/vByte" which is what miners use for transaction prioritization.

Or do you mean the transaction slider's estimation?
Because since it's an estimation, it's not 100% accurate, just close to the target block or mempool range.

It can't be done in the GUI but doable in the command line or console.
Just learn to use send command, then to add an OP_RETURN output, simply add a "data" key with the value of your hex-encoded message.

Example command:

---

You can also manually create the transaction via createrawtransaction command to be able to create a transaction with an OP_RETURN output.
Like in "send", it should be added as the value of the "data" key in the outputs.

Example command:

Since it's a manual process, the fee should be computed from the difference of the input's amount and the total amount of the output(s).
On a side note: This method is where the "or it'll be set a fee" warning should be.

In your Electrum version 4.0.2:
It means that you've clicked "Pay" but if "Advance Preview" is enabled, it'll show you the transaction's advanced details instead of sending the transaction,
And if you close that window without finalizing the transaction, the invoice will be labeled as "paid" even though nothing is sent or locally saved.

What this mean is there's actually nothing wrong with your wallet, just an incorrectly labeled invoice.
Always refer to the history tab since it has the actual transactions that count toward to your final balance.

Yes, that is one implementation of Electrum server.
And I'd recommend it if your end-goal is not to use it for personal use, but public.

Plus it's fork is maintained by same developers of Electrum: https://github.com/spesmilo/electrumx
For the set-up, you can refer to its official documentation: https://electrumx-spesmilo.readthedocs.io/en/latest/HOWTO.html

Yes.

Basically the same as: https://coinb.in/#newTimeLocked
Tick "blockheight", then set 7140000 in the blockheight box below it.
As the matter of fact, it produces the same script which uses pubKey in contrary to others which uses pubKeyHash.

Yours is actually standard in the current protocol but your nLocktime isn't less than the LOCKTIME_THRESHOLD which made it "lock-by-blockheight".
So you'll have to wait for block height 7140000 or the Bitcoins locked in that script cannot be spent.

It's a different scenario this time since they do not have to change anything in Bitcoin to include the "uncompressed SegWit" transaction in their block.
All they had to do is accept that said transaction to their mempool to be included to their block.

Only miners with nodes before BIP-65 implementation may consider your transaction non-standard but valid;
But AFAIK (CMIIAW), the block will be rejected by new nodes.

Unfortunately, the "7140000 locktime" isn't the transaction's locktime but the locking script's.

For reference, here's the input's Redeem Script:

So it's just a simple "fix" to a corrupted block.dat file.

Bitcoin Core already has a feature that can detect corrupted blocks or other data and may start with an error.
Running command line option at start like -reindex (depending on the error) is the usual solution to this, no need to change a line code or anything.

What everyone explained is if you want to change in the protocol, because it is what OP sounds like.

That will still allow Electrum to pick other nodes to connect to.

What he meant is same as the "oneserver" setting or command line option which will keep your Electrum connected to a single node, your selected server.

It still accounts to the wallet's total balance, it's not good to tell the user that it's cancelled.
And that would be more confusing since local transactions aren't all dropped transactions.
Some are simply not broadcasted which wouldn't fit to the "cancelled" category.

Lastly, Electrum has a tooltip that explains to the users that the transaction isn't broadcasted yet, both in Android and PC.