First question, I don't think anyone knows. I've heard warnings about this since I was a kid. It would be really hard to do a study over enough years to really show anything about this. I've popped my knuckles a lot all my life, and now I have arthritis. However, my right knee is my most severely-affected joint, followed by my right talo-calcaneal and my right third distal interphalangeal joint, which yes, I can pop. So if popping is the cause of OA, then how did it end up in 2 joints which have never been popped. There is also the possibility that popping causes joint damage that is different than OA.
A doctor in the states cracked knuckles unilaterally on his own hand for 50 years. No unilateral symptoms. http://www.scientificamerican.com/artic ... k-research