As people have said, I'm not sure in real life walking away is the answer. I would go for 'helpful but firm' - I think I might set out thoughts/facts/potential concrete solutions in calm, reasonably blame-free email to both MOH and bride, so they had time to mull it over, rather than getting hysterical on the phone.
If they then continue to be arsey, I would at that point leave them to it...this is only going to get so very much worse.
Best solution by a long way would be to see if you can get the deposit back - no harm at all in asking.
As an aside, I recently organised a hen do like this, where the bride wanted a weekend away, self-catering with about 20 people asked, many of whom I didn't know, and didn't know each other.
I've wondered if I was a bit too 'organised' with it at times, too meticulous with costs, warnings that there was no pulling out if people had committed beyond a certain point etc. But this thread has given me chills and convinced me I was right...
I think it's a warning that these types of hen dos are very hard work to organise, can go wrong quickly, and it's a massive, massive amount of work to ask bridesmaids to take on.