(1)Bitmain will want to make their holdings look as impressive as possible, so they will probably try to pump bch (maybe the other alts too) before the IPO. They have been trying (and failing) to corner the market for the last year.
(1) Bitmain will probably have trouble pumping bch(they have been trying and failing for the last year).
(2) miners probably won't risk an attack, they haven't up until this point. That said, bitmain & bch are the most vulnerable they've ever been.