No need to worry about cruise ship crowds in Placencia. It stops nearby, but the cruise ship line bought a caye and turned it into sort of a Belize Disney Land theme park. So they all hang out there and don't even venture into the village.
As far as pointers on your confined water test, just remember your training and you'll do fine. Same goes for the open water dives. If you're diving with Splash, they'll take good care of you. Ralph and Patty run a first-class operation. I'm sure you'll be nervous, but they'll help you relax and don't forget to have fun.
Lots of stuff to do on the mainland while you're there. You can rent a car and drive yourself around, but for first-timers I always recommend hiring a tour guide. He or she can do the driving, tell you what you're looking at and will take you to some places you might otherwise miss.