There's no reason why you can't ride flatland, some of the best riders I've seen didn't use freestyle bikes. And I doubt you weigh enough to do any serious damage, there's guys 2x your weight doing it, and it's all in how you land and absorb the shock.

It's tough to say with the bottom bracket. How is it worn out? If it feels loose what you might just need to replace is the bearings. You might need a special tool to get it out to replace them if it's a redline 3 piece crank. I would go to your local bike shop and ask about it.