Buying houses and selling hotels in a housing shortage
There is something about this scenario that seems unusual.  Instinctively, I would say that Alice should have priority because Bob has to first sell his hotels before buying the houses. She should get her 6 houses and then Bob can downgrade his hotels to acquire the remaining.

A poster on this Board Game Geeks thread refers to The Monopoly Companion's description of a tournament rule (created after the 1983 U.S. Championships) that enforces this restriction. 


  While a Building Shortage exists, players desiring to buy the houses
  remaining in the bank have priority over those wishing to break down
  hotels. 


I recommend that thread because they go into other examples that are pretty interesting.

I believe that Bob triggers the building shortage rules once he states his intention and Alice responds.  However, if Bob were to simply state that he is selling his hotels, then it may delay the building shortage and allow for a procedure more similar to the one mentioned by @ghoppe.