If you control one arm with the cross hand on the wrist, the same side hand on the tricept. If he tries to rain down from that position, working the high guard makes for a very easy armbar.
#3, What do you mean both hands on one arm?
As in, Shrimping out and crossing to that one arm so the other one can't reach over?
because, I can see someone raining down on me if i'm holding one arm.
You might get hit once or twice if the guy is dumb, but you'll get the submission. If he's smart, he'll try and pop the arm out and you can adjust and start working for a sweep.