As far as the no shoes goes, I think that is reasonable, esp. since she let people know ahead of time.
As far as the potluck goes, then it is starting to get a bit iffy. Doesn't sound like they really want the housewarming, but are being pressured by people to see their house so they say "Hey, if you want to see the house, go ahead, but bring food".
I think the thing to do for the OP is to say "I am so sorry, something has come up and we won't be able to make the housewarming party. I would like to come by and see you another time. What day would be good?"