All 100% preventable. 

You could tell them if they back out, you will put a lien on the property (not the right term), that prevents them from selling it for the next year or two.

I am not necessarily going to complain because at least I am getting all my money back out but this really negatively impacts my prior strategy.She implied that Bank Examiners are really crunching down on real estate lenders to prevent bank defaults. 

Unfortunately, she'll lose in court - which means her voucher will be lost and she'll have a landlord/tenant suit on her name - preventing her from renting from anyone who runs those background checks.

If I do pay the bill upfront, and then charge the tenants based on my RUBS system, are there regulations that would prevent me from allocating cased on the number of people in each unit rather than the number of bedrooms in each unit?

Money is the only thing to prevent you from holding it off the market and then disappearing.I require a full month of rent within 24 hours to reserve a rental and I will only hold it for 7 - 10 days before I expect you to start paying rent, utilities, etc.

What does this solve for me: it prevents a fine from the local municipality or prevents the local municipality from sending their people to clean up. 

In an article by Brad Beckett: Director of Education & Outreach, National Real Estate Investors Association, The CFPB wants to prevent Foreclosure until the end of the year. 

Is there any legal barrier preventing the QI from just taking the money and run away?

I've had these fights before and sometimes it just takes pointing out the problem the the Tax Collector or the County Attorney to get the situation prevented in the future.