Like you I have seen this question (password doesn’t work) a lot. It’s great that you took the time to formulate a comprehensive response.
I would suggest to login to the email associated with the account (assuming you can get access to that email) in order to see if there are any unexpected logins from new device (which might be an indication of malicious account takeover). I realize that account takeover is probably a much less common scenario than the others (and mentioning the scenario could induce a degree of unnecessary panic in some cases), but perhaps could be at the end of the list.
Edit - I recall there can be more than one email associated with an account, but I don’t remember all the details. Maybe it’s only an option to use a different email if you are using email as a 2FA option.
Edit 2 - It may be that a bad guy who gains control of your account would avoid changing your password in order to minimize the potential of tipping you off to the breach. But on the other hand, he may change your password to lock you out in order to make it harder for you to gain back control of things from him.