When navigating seller financing, what are the typical documents that a seller will ask for in order to verify that the buyer is a credible, reliable, credit-worthy buyer, capable of following through on the purchase?

Good insight.And yah, I’d love to see verifiable statistics on success rate, profitability, etc of going fsbo vs. listing with an agent.

Adding an addendum usually only requires a 30 day to 60 day notice.In addition to that, if you did a pre move-in inspection of the property that verifies that the property was "flea-free" when they moved in and have confirmation from a pest control professional that the flee infestation is the fault of the occupants and/or their pets, I would first, politely make a proposition to share the costs of getting the problem professionally handled.

I'll verify the condition when I visit it tomorrow, and adjust my rehab numbers accordingly

BUT reimburses the wholesaler for the original sale from the initial purchase of the property. 

You can attempt to verify all of this by contacting the providers (water/gas/electric, insurance company, HOA representatives, etc.).Also, make sure you have a couple of potential buyers lined up. 

I should have vetted my GC more thoroughly (trust, but verify).My agent(s) offered Property Management services to me on my rentals.

Most (if not all) will give you a free consultation, which is basically a phone call to see if there's some synergy, if there's an opportunity to work together, and some initial, high-level guidance. 

If you’re unsure of usable commercial space you need to verify.

We did one..we bought the home next door & initially we got $820/mo in a 70's era SFH in very very tired shape.When they moved after 5 years we rehabbed it for $18k & got $1400/mo & that was 6 years ago.We did bump the rents over 2 more tenants (now $1650/mo) but the current tenants do all the maintenance.When the 20 yr old HW tank burst at the seams they shut everything of, cleaned up the mess.