1) Критиканство - captiousness, nitpicking. If you critisize something and your advices are directed to improve it or make it better - you're a critic. But if you critisize it only for showing others that you are superclever or something, and most of your critics is kinda superficial - you're a КРИТИКАН. It's a bit hard to say in two words, though ))
2) Разбираться is a sort of continuous tense. You should use "РАЗБИРАТЬСЯ" for your example this way: НЕ МОГУ БОЛЬШЕ РАЗБИРАТЬСЯ С ЭТОЙ ПРОБЛЕМОЙ, И ТАК УЖЕ НЕДЕЛЮ С НЕЙ ПРОСИДЕЛ
3) The both are correct, I think. Depending on your goals count, of course )
4) ВЗЯТЬ is closer to TAKE, and СХВАТИТЬ is closer to GRAB
6) Догнать in your example means your first variant. To say a second one you should use ПЕРЕГНАТЬ (overtake)

Hope it's all clear enough to understand. If not I may describe it more wider