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If you disable 2FA, your wallet would be still a 2 of 3 multi-signature wallet, but with two master private keys.
Of the 2 master private keys, one of them is for my receiving wallet address, is that correct? and the other one is?
I feel you don't know how a multi-signature wallet works/
The standard wallet you create on electrum is a single-signature wallet which means that there's one private key for each of your addresses.
In a m of n multi-signature wallet, there are n private keys for each of your addresses and m of them are required for signing a transaction.
As I already said, Electrum 2FA wallet is a 2 of 3 multi-signature address. This means that for spending from each of your addresses, two private keys are required.
Since your private keys are derived from master private keys, two out of three master private keys are required for spending fund from your wallet.
Let's say the master private keys are MP1, MP2 and MP3
MP1 is kept is stored in the wallet file.
MP2 is owned by trustedcoin.
Your seed phrase can generate MP1 and MP3.
When 2FA is enabled, you sign your transaction using private key(s) dervied from MP1 and with entering the 2FA code, you ask trustedcoin to cosign the transaction using the private keys(s) derived from MP2.
When 2FA is disabled, your wallet file contains MP1 and MP3. You have two master private keys in your wallet file and you no longer rely on trustedcoin for making transactions.
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If you want your transaction to include only 1 output, you should spend the entire balance of selected UTXOs, so that your transaction doesn't have any change.
So what I need to do is just send the maximum amount of the current multi-signature wallet to a standard wallet, right?
Yes.