No inside access, but it was described as "a hoarding".I spoke with one neighbor who has been inside, she told me there wereintense odors, overwhelming stockpiles of clutter, animal feces, bugs, even rats. 

You can clean the walls with TSP, use an ozone machine (not exactly necessary, but work well to remove odors), use a good oil based primer like BIN or Kilz, and then paint the walls at that point.

Some Kilz odor blocking primer, a coat of paint and LVP throughout.  100k profit!

Short of suspected foul play I can see no reason anything would tie up your property post mortem, and even then it would be brief.  

A tenant just moved out of my rental property and there is a urine odor throughout the first floor. 

I care about my properties and having less damage, and no forever odors.

Meaning, how do we prove there was an overwhelming odor that we had to address in order to re-rent the house?

The odor of cat urine remains.

Do your sellers have stubborn pet odors/ stains that won't come out?

Do your sellers have stubborn pet odors/ stains that won't come out?