In case it helps others, here's the outcome. No charges and no issues. Despite the online reservation still saying I needed to drop in MIA, the lady at EYW airport took the car, checked things in and processed the final bill with no additional charges. She commented that she saw it was originally supposed to be dropped in MIA but saw it changed.

I think IAHtraveler is correct re: the FL statute that prohibits one-way / drop fees. Not sure if also being a Fastbreak member helped in this scenario.