But unbeknownst to me the property had a foreclosure sale date two days after the contract was submitted by the realtor and subsequently was sold at auction for the exact price I was selling it to my end buyer.

However, if you need to cut into the joists to bury the beam you'd be looking at least a solid days work.We subsequently drywalled over the beams & that was about 2 hours.

In subsequent years, you're looking at 36-50% CoC ROI on your $25k investment.

You need to inspect the unit, take photos, etc to document the current damage/condition so that you can charge them for subsequent damage at move out.

If that doesn't work, I suppose the next step is a formal letter advising them of the previous personal conversation and lack of subsequent compliance, with some kind of date for compliance.

Not to sound like a motivational speaker (I'm not) but subsequently much of what they say, we as successful RE investors put to practice.  

I have heard of few troubling stories of bad tenants or bad neighbors snitching on the landlord for one reason or another and the stove is subsequently taken away and the kitchen ripped out. 

I think you need to separate in your mind the note purchase and any potential subsequent modification. 

Since they contacted you on Airbnb, take at least the first month's payment there, then subsequent months you can go off platform. 

However, they were on a nonsmoking lease (with the prior owner as well as the new lease with us), and we discussed the smoking issue with them when we purchased the property last year, and also sent a subsequent written warning which we received back with a very hostile and crude reply.