The marine combat could take place as a boarding action or a defensive action instead of there being a ground map at all. Imagine those three UPP fleet subs launch boarding torpedoes. The Almayer is subject to hall to hall fighting as the UPP try to take the ship. If they manage to take CIC/engineering for five minutes then the UPP override the safeguards and flush the atmo or overload the reactor, resulting in an opfor victory. Otherwise the tattered remnants of the marines run their own boarding action of a damaged UPP sub. It was caught on the edge of a nuclear blast and its systems have been disabled. It's in a timer though, and the UPP sub is able to recloak and slip off after thirty minutes or so. Alternatively, if the ship is basically lost but hasn't been fully taken by the Marines the UPP captain can blow it's reactor after thirty minutes.
Marine major = capture UPP sub
Marine minor = capture UPP codebooks or key UPP personnel but the sub gets away. Alternatively, if the sub self destructs it's a marine minor.
Draw = the UPP sub gets away
UPP major = the boarding parties capture or destroy the Almayer

Spice that up with high command announcements as ships on both sides are destroyed (especially other USCM ships destroyed by boarding torpedoes) and that's a fun time.