"Is "suck it up" the only way if one does not want  to explore a gray area of a) playing with dates b) declaring incorrect status in MN return"

When Minnesota says "Jump" and your client asks, "How High?", you just move on and try to help the next client who is more pit bull and less sheep.  

In many cases you can file it their way and then file an amended return claiming it your way.  That might provoke them into admitting they are wrong.  But here they would just say you can't amend joint to separate.