I've called the Le Mederian and asked if they could remove the stay about a week+ ago, only to be told that since 90 days had passed, they were unable to modify the stay from the rewards side of things.

I agree with you 100% -- requesting a credit for a stay shouldn't be this hard, nor should it require me to literally call Marriott Customer Care (to the point where Tim remembered my voice, my case, and said to me today, "This still isn't fixed?") at least twice a week. It should not take multiple phone calls, it should not take multiple emails, and I should not be getting rejections only to be told, "If you have documentation related to the stay, we're happy to review it." Do they think I seriously asked the property I was staying at to provide me written documentation saying that we were walked, then expected me to hold onto that until my stay credit posts, 3+ months later?