1. There’s no reason to make him the Chazan. He should keep up with the Chazzan until after the second bracha and he can say kedusha together.
2. It seems to me that saying eloki ntzor as the shatz is not appropriate because it’s delaying the tzibur.
3. the reason many chassidim skip tachanun many times has to do with kaballah. The Arizal says that if you say nefilas apayim if you go down to the klipos and you’re not careful it can be quite dangerous spiritually. This is likely the reason that people say perek 6 instead of perek 25 like the Arizal did, but it’s mashma from the Arizal that it’s not dependent on the specific perek you’re saying. It’s better not to say it so much than to say it, go down to tumah and not know how to raise it up and end up getting mired in tumah. I’m sure the misnagdim will disagree with this, but nevertheless, this is what it’s based on.