The cost of this will likely be several grand

I've written several BP articles on these topics as well.

Considering possible interior and exterior damage from the large dog of the remaining tenants, and other damage that may not become evident until prepping house for next tenant, how can we handle the refund of security deposit to MIL couple, since, per the text of the lease: “All Lessees are jointly, severally and individually bound by, and liable under, the terms and conditions of this Lease.”?

I dealt with several.

I don't know whether this is typical, but my experience is based on several years buying foreclosures, and my wife as a VP for a finance company with non performing loans, many of which were real estate.First, for the foreclosures I looked at, most if not all were at public auctions.

I have been looking into several strategies to pay it off and one of my first cousins (who is an international student in the US) has saved up $15k and would like to partner with me/ lend me the money to buy the rental.My strategy was to use his money to pay off my 401k loan balance and then do the roll over into a self-directed IRA to buy the rental under my name so that he has the option to live in it (if it's a multi family) and rent the rest.I have done a great deal of research and it looks like as long as I don't live in the rental and also make sure that all the transactions are done within the IRA, then it should be straightforward. 

I'm looking at a property now that has been mis-managed by some adult siblings, who received the building after their parents' death several years ago.

Drafting and revising so many loi's, binding and non-binding loi's, poc's and several different other documents. 

Part of my contingency is seeing the rent roll over several years, and ensuring things are up to code.They haven’t explained why they can’t provide but the selling agent said they will have a hard time because it a mom and pop shop

I think this area is going to be attracting people and jobs for the next several decades.