I was approached by Avantstay and wondered if anyone in this forum has had experience with them from owner point of view. 

I'm not the fair housing landlord expert but I'd probably steer away from referring to it as "veteran housing"Either way I can relate to your situation a little because I also own a house that is shared by multiple people.

Have you ran this by your landlord/tenant attorney?

What changes did others incorporate into their leases that were influenced by this years tenant interactions? 

I would be surprised if there weren't notices given to both the defaulting End Buyer (You're in default, you must close by X date or we're forfeiting your deposit) as well as the escrow agent (Buyer defaulted, see contract Earnest Money paragraph regarding release) prior to that actually occurring.

Does the contract require arbitration or remediation?

@Garry LawrenceDevin Moreno has a bunch of videos on rent-by-the-room strategy in Baltimore on hisYoutube channel.

It was listed by David.

It "was remediated with cap in place so there is still contamination at depth".

Disclosure laws vary greatly by state.