By the way, I went ahead and uploaded that aforementioned spreadsheet as a Google Doc.

This is it: all 330 Marriott properties in the US (50 states + DC) currently classified as a resort or convention hotel, either by Marriott's main site or their Marriott Bonvoy Events site.

If the Marriott hotel you're staying at is in the US and isn't on this list, then you should be good to go for guaranteed 4 pm checkout - meaning if they deny you, they're in violation of the program.

This may be true, but the way I look at it is, then they get away with it. And if they get away with it this time, they'll keep doing it, knowing they'll get away with it next time, too. (I don't personally ever see myself refusing to leave if denied late checkout by the hotel; I'm more referring to the "hours on the phone with Marriott" part.) Maybe that's petty (I don't think it is), but I care about getting what I earned and paid for - and you can feel good about yourself knowing that you didn't just help yourself, but others who will be in that situation in the future. (And in theory, once you have that first case number, you can keep referring to it; it will already be documented that the 4 pm checkout is guaranteed, so the agent won't be able to deny it, and it should all get resolved pretty quickly rather than taking hours.)

I mean, someone's gotta police these hotels and hold them accountable if Marriott's not gonna do it on their own. It only takes one time for them to learn that there are consequences from corporate for denying guaranteed benefits. Otherwise, as long as there's no consequences, they'll keep doing what suits them, at the guest's expense.