You should use the format p2pkh when recovering in Electrum.
That is true, but it is not necessary. The private key would work as well as adding p2pkh: prefix to it or not. But if the address is starting from 3 or bc1, that is when their prefixes are needed. For legacy address, it is not necessary.
Remember that you probably have a lot of money in bitcoin forks as well.
After you have sold your bitcoins at coinbase, you can try to recover your Bitcoin Cash (BCH) and bitcoin SV (BSV). They are worth some money as well.
There are many wallets which support those coins. But remember, move your bitcoins first (as they are the most valuable coins you have)
You are right. I think this guide should still be of help:
LoyceV's Bitcoin Fork claiming guide (and service). But I hard that Coinomi is having issue. I am not sure about this. I have stopped using the wallet since 3 years ago.