The house I recently bought (cinderblock foundation) had moisture on the walls in one corner which we noticed during the home inspection (it had rained the night before).

Stuff like moisture invasion does NOT get cheaper by waiting to fix it.What you're going to fight is a buyer thinking you're trying to sell services and over-emphasizing some problems - It's not you, just buyers.Only thing I'd say from looking at similar things is if it's a 1-page report and 10 pages of ads, that doesn't go over well. 

It is also a mixed bag of results regarding using any underlay and moisture barrier. 

I've bought and flipped/rented in Florida while living in Nevada (among other states as well), and each location has its differences, not a lot of moisture in Nevada, but tons of it in FL as well as termites. 

On a walk-thru as a buyer, main thing I tell people to look for is moisture invasion since that can be expensive and needs to be fixed sooner rather than later. 

Foundation - cracks, moisture in basement/crawlspaceThere are plenty of smaller items but if you're getting into BRRRRs, these are the four major ones that could make or break a deal.

There is a moisture barrier between the two layers.Question #1 - Is it typical for there to be two layers of the subfloor or am I describing it wrong?

For example: was it caused by poor grading/drainage, tree roots, post-tension cable failure, water leak, poor moisture management (i.e. no sprinkler)? 

Your worst enemy here is moisture, so yes make sure your HVAC system is working properly and make it clear to the tenant that they notify you of any dramatic changes to the floor or noticeable water damage.

It is important for me that the whole system is checked properly, and I can be sure there will be no surprises like leaking valves or moisture in the basement.