First look up the routing rules within the fare rules. It may say that travel must only be via DTW. If not, then you will want to use the multi-city function and specify DAY-MSP, MSP-BOS. If it is a legal routing and there are seats in your booking class, nwa.com will price it out correctly.

The same should be basically true for the second example. Use Multi-city and search by schedule. If there are W or A seats you should be fine.