The building's Property Manager is sympathetic to the owners at large, but says they can’t do anything until instructed by the Board, and they indicated the building’s lawyers would only respond to Board members.

They are on the market longer and are not going at asking prices but some people are still being very aggressive with their asking prices so it really isn't a great indicator.

A delinquent tenant on month one is indicative of poor screening. 

I'm also seeing lots of homes for sale that were used as Airbnbs, so I'm wondering if that along with these numbers may indicate it's an overly saturated market, and if these are signs that maybe I should keep searching for a stronger market, or just a property that cash flows given the average annual revenue?

If the only indication that he is difficult is not wanting to send his ID to a stranger on Facebook, I wouldn't blame him. 

Recently, my mother has indicated her wish to sell the family property where I grew up in Isabela, Puerto Rico.

Here are some indicators to help you assess a buyer's seriousness:Questions: Serious buyers will ask detailed questions about the property, such as maintenance costs, neighborhood amenities, and property taxesStrong Offer: If a buyer makes a strong offer quickly, it often indicates a high level of interest and intent to purchasePrivate Buyers: If approached by a private buyer, ensure they provide a Letter of Intent (LOI) and proof of funds.  

Normally the escrow agent will want mutual sign off even if its clear the contract terms indicate one party should be awarded the escrow funds.

Are there specific market indicators or property traits I should be paying attention to?

Unless the medical condition meets the state criteria for early termination and that the medical notice does not appear to suggest that, you cannot terminate the lease outright/immediately but that you will begin to market the property for rental and upon securing an acceptable tenant, the lease can be terminated pending a satisfactory property walkthrough indicating no damages.