Two separate issues here:  Title and loan qualification & terms.Title:  If this were my chore, I would create a Living Trust in the parents name and appoint you and your husband as the trustees with full POA for all financial and property transactions(the lawyer should provide an Abstract of Trust). 

They've only been in the house for 6 months and have consistently tried to pay late every month (I have insisted on on-time rental payments, and they have complied, but grudgingly), brought a dog in despite a "no pets" policy, and are generally stand-offish (the first thing they did when they moved in was install a security system, and when I come by the house even with an appointment, they call me while I'm on the front steps and say that they're not in the house). 

(a) If the owner's interest in the premises is terminated by sale, assignment, death, appointment of a receiver, bankruptcy, or otherwise, the new owner is liable for the return of security deposits according to this subchapter from the date title to the premises is acquired.

I liked making individual appointments, as it says a lot when someone shows up on time or not.  

And, even then, try getting a counseling appointment if you're not willing to pay for it.Or at least, if your definition of "relationship" business equals charity, then why charge rent at all?

Should I know whether this is a good seller to make an appointment with, based on the ARV, and what he wants for the property?

Finally- your state should publish a landlord/tenant guidebook- those are a great resource to give landlords an idea of their rights in your state, every landlord should be familiar with that book. 

You understand it's a hardship for them to have to drive to medical appointments, etc., but if they don't make paying rent on time a priority, they'll end up evicted, and if you have to come fly there to take care of problems from Alaska because they aren't paying rent, you are going to have to hire a lawyer and evict them and get different tenants. 

I had one lady come into my office before we went appointment-only and scream at me that I was a horrible person and how evil capitalism was for America.

Make an appointment and go visit the office to address your concerns, it may be worth it... every housing office is different, some are easy to work with and others just are not.