If we entertain the possibility that your account got hacked and you're not the one sending those PM, wouldn't the content of the PM itself no longer relevant and won't help with your situation, as your primary goal here now will be to prove that you got hacked?
What difference does that make? There is nothing in the contents of that PM that can help you.
The content is relevant to me as I could maybe make a clue out of it about the intent of the person who logged into my account.
Relevant how? You happen to know the hacker or will be able to identify the culprit if you get ahold of those PMs? And about the intent, err... It's to phish? Kinda moot.
Also, I am sure I can prove that my account was hacked and it was not me who sent those spam PMs by showing the proves of that password change email with hackers IP.
You're free to provide the list of IP logged in to that account here and let mods decide if they'll still stick to the ban or not.