Mahahual is up and running completely fine, following Hurricane Dean in 2007. There’s a new beachfront pedestrian promenade, and lot more building. There is an ATM, but it was not working when I visited. One of the hotels was offering to give cash for credit-card charges, for a 10% fee.
Caribe buses no longer go out to Chetumal via Xcalak. They start in Xcalak in the morning, and come up through Mahahual, then head out.