I dont think this is a good idea since addresses should be generally only used once. And additionally this leads to centralization of mining.
Big miners get higher probabilty of mining the next block.. thats a catch-22.
Thats basically like the first point? I don't see a considerable difference. Leads to centralization and not the way bitcoin is intended to be used.
There is nothing which speaks against miners who mine on different chains simultaneously, as long as its PoS.
Its kind of hard to get miners stop switching from BTC to BCH mining and vice versa. The easiest way would be for one currency to use a different algorithm.
But since BCH follower think its the "real" bitcoin (until it crashes to 10-200$.. which very likely will happen) there won't be any updates to mining algorithms.