I use my left middle finger for M and M'. Push forward on BD for M' and push up on DB for M. I don't get how you can use your index finger for M, I don't think it's fast no matter how you do it. Pinky for M, wtf at that too. Fingers not big enough and it's painful to stretch that far.
I think if you're right handed, doing M/M' with one of your left fingers will be easier, because you have a much firmer grip with your right hand, so the layers won't misalign and you won't lock up.