MSP and SEA are but two of the examples where IM is used for the 1200 DH case. As you say, there is terrain leading to the aproach end that is unsuitable for RA use.
For the 1600 / 150 case, I found the answer in Appendix A of Part 91. It states that the barometric altimeter may be used "no lower" than 150 feet to determine DH. So, AT 150', use of the baro is legal.
Thanks for the reply.