... the fact that we have 5 different private keys,...
Hive accounts come with 4 private keys only! It's a bit disturbing to see you make the same mistake over and over in the many posts that come to promote your onboarding process. I hope this one doesn't end up in your educational courses.
Did you forget about the Master Password? I personally still consider that a "private key" that is very important to secure! I hope you didn't forget that.
The Master Password is not a key but a seed that can be used to derive the 4 key pairs for an account!
Considering it a "private key" implies that there should be a "public key" counterpart, which is not valid.
Add to this that you cannot do anything at the blockchain level using the master password.
I agree it's important to keep it secure, but naming it a "Private key" only adds to the confusion.