About 2: I remember in the past there were errors if multiple .cer were present in the keys folder, I see now is ok, thanks.
About 5: The Firebase console also inform the certificate is expiring, why you say this step is not needed ?
About 6-8: ok
You are correct. I thought that you meant to something else.
Note that you can also use the new key format with Firebase. I don't remember the exact terms however you just need to download the new type of key and upload it to Firebase.