1) When does the miscast happen? is it after the spell is cast with irrestible force or when the result is actually rolled on the miscast table? 2) Do I have to use cupped hands on my first miscast? or can I choose when to use it? I think waiting till you have a real nasty miscast then giving it to an enemy caster (perhaps along with bane head) would be very good!
After the miscast dice is rolled, you can choose whether to transfer it or not. Worth taking a few until you have a really nasty miscast? Maybe. It is a tossup; how many miscasts do you expect in a battle? You could use up Hands in the first turn on the weakest miscast, then next turn get sucked into the warp. Conversely, you could cop a wound or two from a mild miscast then not have another one the whole battle.