SegWit and SPV-mining. What if...?
Yes, they can do that.
Correct, though the switchover requires a 95% threshold to make the chance that old nodes see confirmations on such blocks very small.
Segwit nodes never accept blocks that have a witness commitment but no witness section. Either the block must not have a commitment (in which case it will be invalid, as script validation fails), or it is outright not accepted (because there is missing data; similar to a block with missing transactions isn't accepted).
Miners that don't validate a block before mining on top of it violate the security assumptions of the network. We can't prevent them from doing so (with or without segwit), but that is one of the reasons why block propagation and block validation needs to be low-latency: so that the potential advantage miners get from SPV mining is not too significant.