We just got back from our trip and we went through Mexico. We got the bus from Playa to Chetumal, and we stayed over in Chetumal because we wanted to look at the Mayan Museum, but we didn't need to do that, we had plenty of time to get across the border and fly from Corozol to AC.
You can arrange to be picked up at the bus station in Chetumal and taken across the border:$30.00 US, and well worth it. We called Henry Menzies, BelizeTransfers.com. If you get a morning bus from Playa,they will get you to the airport in Corozol in lots of time.I think the bus ride was about 4 hours.
There are two bus stations in Playa, the one you come in to from the airport, and another one about a mile away. It is the other one you get the bus to Chetumal from. Get your tickets the night before, the busses are full
By the way, if you come through Mexico DO NOT LOSE THE PIECE OF PAPER THAT THEY TEAR OFF THE LONG PIECE AND GIVE BACK TO YOU. KEEP IT IN YOUR PASSPORT.