It's not enough that MSP isn't specifically excluded; you actually need to see that MSP is included in the routing, which would look something like this:

ROUTING 509 FROM-TO IND-NW-DTT/CLE/EWR/CVG-NW-PHX*
FROM-TO IND-NW-PHX*
FROM-TO IND-NW-CVG/EWR/CLE/DTT-NW-STL/
SDF/RHI/PAH/MSN/MKC/MKG/LSE/LEX/LAF/
GRR/GRB/FWA/DAY/CVG/CMX/CLE/CWA/CMH/
CHI/ATW/BNA/BHM/ATL-NW-MEM/DFW/HOU/SLC/
MSP-NW-PHX*

Also, a "surface sector" is travel from one point to another via something other than an airline. It doesn't have to do with the "surface area" enclosed by other points. So it would have something to do with, say, an "open jaw" (e.g. BOS-DEN, return COS-BOS; DEN-COS would be a "surface sector" since it is not being traveled in the air). What I don't know is what a "fare break surface sector" would be - how could you break the fare at a point to which you're not traveling by air? Perhaps this rule is a complicated way of saying something like "no open jaws allowed"? But normally if that's what they meant, that's what they would say.

You also haven't mentioned that you confirmed that the DAY-MSP and MSP-DTW flights had availability in the fare class you were trying to book. As I mentioned before, that would be a requirement in order to book it, even if MSP is allowed within the rules.

Hope that helps.