How do you prevent this from happening?

Him not following the swimming pool guidelines alone and causing you un necessary repair costs should be grounds to evict him, but again depends on what your lease saids are possible consequences and your state laws are. 

Make sure it's structured right with an attorney to prevent that from happening?

Is there a way to validate info like back taxes, liens, other issues that may be attached to the property that would prevent me from taking over?

Meaning, send a letter to everyone involved in this property of intent to refuse to sell, or hire a building inspector to find faults, or other such actions to delay or prevent a sale?

Some towns have brutal rent control ordinances that can prevent you from increasing your annual rents. 

This would not prevent your debt to income ratio from being too high, because the mortgage would initially be in your name the banks will still view this the same.Lastly, the six figure bracket is not always as easy as it seems in real estate because things will come up that you completely did not expect and take a huge chunk out of what you're expecting.

If the NOD has been filed and the property is going through it's listing phase in the local paper leading up the the auction date are there any impediments that would prevent it from being a viable (assignable) deal?

Anyway, I don't have short sale experience but I can't see why a seller could prevent you from doing whatever you feel like doing with a property once you own it. 

No piece of paper can prevent hurt feelings if the situation turns bad.