Is the procedure for my 90-day report and the fine I received correct?

Yep, totally correct procedure/fine.. (y)

You can file a 90 day report from two weeks BEFORE the due date until 1 week after the due date without a fine. So the fact that Immigrations was closed on your due date cuts no ice to them as you had a 3 week window to file.

Once you're more than 7 days late you do get a 2000baht failure to file fine when you do your next report. Good news is that fine doesn't increase whether you're 8 days late or 8 months late, it's just 2000baht.

Now as far as could you have filed at an office other than where your primary residence is, that's between you and the office you were trying to file at. They don't have to let you file, BUT they can if you prove you're registered as staying there (TM30 from the hotel) to show you are at an address served by them.

Live and learn, you have 3 weeks to do the 90 day reporting in person.