I like think bars for bench pressing and sometimes military pressing. I'm stronger with them and they are easier on the shoulders for some reason. I don't like them for pulling or biceps... YES I understand that you get additional grip/forearm work BUT you also have to use less weight so the main muscles end up being underloaded. I do, however, like thick bars/grips when working grip specifically,
It depends. Few people can do a solid/efficient squat jerk. When training athletes I teach both the power jerk (also called push jerk) and split jerk. If I had to pick one it would be the power jerk which, as the name implies, is better to build power. However it is more difficult to get into the proper catch position (bar behind the ears, hips back, head and chest pushed forward).