Hotels cannot modify points reservations. Only Marriott CS can do that. I can’t comment on who determines availability.

OP, I would try to HUCA. I had a similar situation where I needed to change a 2 night pts reservation to 1 night. No idea why I booked 2 nights as one reservation since I know better! I couldn’t modify the reservation myself as there was no availability. I called CS with very low expectations as I didn’t think they’d be able to help either. I had a very competent agent as he was able to change it from 2 nights to 1. It cost me a few thousand pts more than my original rate, but I was still happy with this outcome.