How are conflicts between pilot flying and pilot not-flying over aircraft control resolved?
The whole idea behind the policies and procedures for transferring control of the aircraft between the pilots is so that everybody is always on the same page.  Somebody always needs to be flying the aircraft.

If I want to hand control over to the other pilot, and he doesn't want it/can't take it, he would simply explain why not.  I have never seen standard operating procedures (SOP's) that explain how to refuse, as simply saying no or unable should let the other pilot know that there is a problem and should be the beginning of a conversation about it.  On the other hand, the SOP's do cover the scenario where the other pilot doesn't respond.  

This is why there is always a challenge/response as part of handing off control.  If the other pilot were incapacitated and I didn't realize it, I would say "You have the flight controls" (or whatever phrase that the SOP's require) and wait for his response.  If they didn't respond, I wouldn't just blindly stop flying the airplane and assume that they are.  Instead, I could say it again or look over at them to see what was going on and figure out what the problem was while continuing to fly the airplane.

Now, to request control from the other pilot (rather than requesting them to take control), I would simply skip the first part and say "I have the flight controls" and they should respond with "You have the flight controls."  Again, it's a challenge/response situation so that everybody is on the same page.

In either case, the captain is the final authority and can always override a request.  Airline policies are all very clear that if there is a disagreement, the captain has the "tie breaking" vote.