SSH treats the key as password, you can have the same authorized key associated with multiple accounts.

Not sure why you choose to look at the key as identity.

SSH treats your key as evidence of your identity.

You can choose to give the public half of your key to multiple entities to allow them to verify your possesion of the private half of the key but that does not translate into multiple identities.

Not sure why I can't get my point across. It's simple really:

Can you use the same ssh key to ssh into different accounts on the same system?

Ah I see, yes, you're right.