I think you have done as much as you could and probably way more than most people would have done. It is clear that you are not going to leave this incident feeling well treated. That is a shame.

At this point, I think you need to stop wasting your valuable time on people who won't take responsbility for their actions (or their agents' actions). You have at least four choices:

1. Suck it up and continue staying at Marriott's;
2. Stay at your favorite Marriott's but move the rest of your business elsewhere;
3. Letting them know that you will be withholding $X thousands of dollars of business before you will return to any Marriott property; or
4. Write off Marriott entirely as a matter of principle and letting them know that you will be doing so (but you really need to stick to this).

I like Option 3. I've done this once in the past where I was out $X due to something that a vendor (to whom I was quite loyal) should have corrected and was having difficulty stepping up to do the right thing. I estimated the margin that the vendor got (very low in their industry) and told him that unless they refunded $X, I'd be shifting all business to their competitor until I'd supplied at least twice that margin $ to their competitor. Then I'd reconsider which vendor to use. $X was not big but at 2% or 5% margin (or something like that) the revenue loss was meaningful. I got an immediate email back saying that they'd credit $X to my account.

But, whatever you choose, it is time to let the thing go emotionally.