In my case, I have residential rentals in one LLC, commercial properties in another, self storage in a third, and my real estate company operates in a fourth.

I think you will run into a related party issue rather than an LLC issue in regards to the 1031 exchange. 

A realtor that you are wanting needs to be new and fresh enough to the have energy and willpower to work their a#$ off, but also have some experience and education to bring to the party. 

In general, the major brokerages are somewhat greedy and they would prefer to keep both sides of the commission "in house", so before any new listing is posted to a site like LoopNet, Costar or Crexi, it is passed internally to every other broker in the office and then within the intranet for that company, to see if anyone has a party to do the deal with.

I wouldn't rely too much on airdna or any other similar third party.  

The seller will still be responsible for paying taxes on the interest income they receive, and when the principal is eventually paid, there may be tax implications for both parties, depending on how the deal is structured.

Petersburg, FL in third. 

I've worked exclusively with builders on for-sale subdivisions throughout Southern CA for the past ~20 years (thousands of new homes) and cannot think of a single time one of my builder-clients has retraded on price after a third-party inspector toured on behalf of a buyer.

In other words, when the association responds that a certain amount is due, it is “estopped” from later claiming some other amount is due, because the parties have relied upon the numbers provided

And that's the exact scenario here.If you search BP you will find many examples supporting the position that an investor should never allow a party facing foreclosure, default, distress to remain in a property after settlement.