1. In ghost mode / GM - On Global Risk side you need good headphones to hear the ghost or if your on Black List side you need to learn to bunny hop / bhop.. look it up on Youtube if you don't know how and try getting a better knife weapon as in (Kukri,Axe and Shovel etc.).
2. The guns you mainly wanna go for, well if your a sniper I would try to get awm real fast.. but if you rifle I would suggest K-2, M4A1, or AK-47 and also try to get Desert Eagle comes in handy.
3. I am not sure.. I don't FPS lag on Crossfire.. sorry