I think the IK problem with the arm means that the arms are not mirrored; They are not positioned exactly the same. I usually set up the bones for one arm then mirror them over to the other side. You'll find that staying totally symetrical with your models will save you so much time and head-ache in the end. You can break symetry after the model is rigged.

As far as making the hand of one character "stick" to the body of another character; you might try setting up a simple skin cluster on the model's chest then point constrain the other character's arm IK handle. You could also choose a bone or any other object or locator to constrain to as long as you set the constraint to "maintain offset" When you turn off the constraint, the arm IK is free to do what it wants but when you turn it on, it will stick. Now, you don't just want to turn it on . That will cause the IK to "snap" to the chest. You want to keyframe the constraint changing from 0 to 1 over time. You can turn it off when you're ready for the IK to move away through normal keyframing.

I don't want to give you the impression that this is as easy as what I just explained. It requires that your rig is set up properly and that you know how to effectively animate everything but, it will work.