Does anyone take this info into consideration when looking at a rental market or is such analysis unnecessary?

The absolute best way to avoid taxes is not earn income.  

How can one do basic homework to avoid making expensive mistakes?

Creative financing can help sellers make more money, solve pain points, gets them the money they need, and help them avoid a high tax bill.

Lots of bad reasons.Full disclosure - I give advice all the time to avoid companies that require you to close with cash.  

Since you’re considering converting the upper unit into a short-term rental, you may also want to implement strict rules around smoking or strong odors in your house rules to avoid future issues.

I had already sent a certified mail with return receipt which she is avoiding.

There isn't enough information in here about the current state of the house to be able to advise you, but I would think #3 is a definite no as it seems unnecessary from a cost perspective although that market has scale to accept it.

The owner was motivated due to personal circumstances and wanted to avoid the hassle of listing the property publicly.

I’d avoid flat-rate fees if possible.