Alright, so, bone and muscle strength are obviously a must. Though if you have inherently brittle bones, I wouldn't recommend tricking to begin with. Muscle strength is handy, but you'd want to focus on core strength to get most rotation-centric tricks pat down. Leg muscles too perhaps, if you're a little thick on the side and still want to get in on that tricking action.

Depends on what you consider enough. If say, you want to do aerials and not fuck up most of the time, you'd want to be able to do a rather wide split, etc etc.

mmmmm, i'd say yes based on experience, but you'd want to do your own research on that

One of the best places to test your flexibility and learn how to do rotations and the general stuff are those activity places with trampolines with foam underneath and bouncy mattresses. I'm not a trickster that does those tricks on ground or on floor(I have never tried) but I sure can do some crazy tricks on those trampolines with foam underneath or on bouncy mattresses and most of the time I land on my feet(having a good body control) I'd recommend you to start off tricking by practicing some tricks in those activity places with bouncy mattresses or trampolines with foam underneath. The most common thing what makes tricking diffucult to begginners is fear. Fear of not succeeding to perform the trick and hurting yourself, sure there probably comes that but you have to get over it and try.

I honestly think that bone and muscle strenght everywhere but in legs are unnecessary but optional to have.

I really, really do not consider myself a flexible or very strong person. I can't even touch my toes, I reach a little over halfway down my shins. I bench, at most, 90 pounds. Yet, I'm able to do quite a few lower-level tricks, such as backflips, frontflips, websters, and bkicks. And a few more on trampolines.

The point is, you don't necessarily need to be super toned or flexible to start tricking. Hell, the more you practice it, the more you'll get in better shape anyway. Just stick to trying not-so-insane tricks, get over the initial fear, and focus on technique. It's fun, and overall worth it.