The option is in the setting icon (upper-right) in the "New Transaction" window (window shows after clicking "Pay...").
It's ticked by default.

It's a set of addresses that have different derivation path than your receiving addresses,
Electrum reserves those for receiving the "change" of your transaction: excess amount from the input(s) and the recipient(s).

The wallet isn't specified but I know one wallet which is notorious of giving that specific error, Blockchain's online wallet.
Sadly, those who experienced that error had to contact their customer support or import their recovery phrase to a wallet that supports BIP39 seed.

Please consider reading this sticky topic before asking for support: https://bitcointalk.org/index.php?topic=1741772.0

Is the wallet where you received the bitcoins a "watching-only" wallet?
Did you make the bitcoin wallet (not address) yourself or from somebody else?

Because if is it, someone is trying to scam you.

This is how to achieve that:
Create a new wallet with your Ledger's seed phrase on the same Air-Gap machine, then create a watching-only copy on an online PC.
Create a new wallet with a new seed phrase on your Air-Gap machine, create an online watching-only copy of this as well.
Transfer the bitcoins from the restored (Ledger) Electrum wallet to the new cold-storage Electrum wallet.

If you don't know how to spend using the air-gap/watching-only pair, follow this guide: https://bitcoinelectrum.com/creating-a-cold-storage-wallet-in-electrum/

The guide has the necessary steps on how to import a BIP39 seed to Electrum (it's good given that you'll do it in an air-gap machine),
but the "Detect Existing Accounts" option wont work in offline Electrum so you'll have to manually set the script type and derivation path.
If you have multiple accounts with different address type in your Ledger, you'll have to restore those as new separate wallets in Electrum.

One issue I can find here is since you'll use your seed phrase that's generated by Ledger, you won't be sure that it's now out of Ledger's eyes.
You should instead create a new seed in the same air-gap setup and send all of your Bitcoin there.

According to ChainQuery, it's available since version 0.17.0.
I did not research to confirm it though.

Reference: https://chainquery.com/bitcoin-cli/createwallet

If you have a fully synced Bitcoin Core with complete blockchain (not pruned),
you can create a watching-only descriptor wallet and import all of your master public keys with importdescriptors command.
You can still do that with a pruned blockchain though but you'll have to resync from scratch when you need to rescan the blockchain for balance.

Here's the guide on how to do it: https://bitcointalk.org/index.php?topic=5399337.msg60178631#msg60178631

If you're limited to Electrum, then it's not possible since it can't accept multiple master public/private keys/seed.

Yes, the mining commands generatetoaddress or generate do not automatically run since it's only recommended to use in RegTest.
And AFAIK, even in the old versions, the "Generate Coin" option wasn't toggled on by default.

Try this: https://docs.lightning.engineering/lightning-network-tools/lnd/unconfirmed-bitcoin-transactions
Go to the "Incoming transactions" part, there's a part that includes a guide on how to do CPFP.

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.