1. Yes. $18.50 I think, and you definitely have to pay it even if you're just transiting Belize. It happened to me last January on my way back to Cancun from Guatemala. I'd spent a week in Belize on the way down, and didn't mind paying then. Coming back, they hit me again. I wasn't overly impressed with Belize during my stay there, and getting nailed again to spend a few hours on a bus in the country left a really bad taste in my mouth about the place. The border people in Corozal were total assholes about it too. Belize has nothing that its neighbors don't have more of, usually for less money too, and I won't be going back any time soon.

2. In theory, yes - probably in about 27 hours with connections, if you really push it and time things right. That'll be the longest two days of your life though. The tourist buses from Guatemala City to Flores take 8-9 hours, and you can make it from Flores to Cancun in about 16 hours. Tourist buses from Flores leave at 5 am for Chetumal, and from there it's another six hours to Cancun. You'll get hit again crossing Belize (the fuckers), even if you never get off the bus.