Bitcoin mining has been custom ASIC dominated for years now. Almost all miners (in terms of hash power) are custom purpose built machines. So no, they really can only do sha256 hashing on payloads of comparable size to bitcoin blocks.
Which might make them harshly powerful password crackers for anyone daft enough to use SHA256 as a password.
Except that BTC does a double hash, so it won't work on any 'regular' single hashed password.
Nope. GPUs can be repurposed but the asics that are used for Bitcoin mining are expensive paperweights without demand for BTC.
Well they are relatively good space heaters at least...