Would a CPA letter suffice to prove doing business for 5+ years?  

Merely stating that you want the value lowered to a specific amount due to condition, should suffice.

My attorney said it was not necessary to buy property in the name of an LLC; that getting a high enough insurance policy would suffice?

A paystub or W2 will suffice though (as well as an employer reference)

@Carlos LopesThere are several considerations that can go into the analysis of whether you need an LLC or whether a large insurance policy will suffice.

Since they are unable to travel to the U.S. in time for closing, should a notarized document suffice for the title company?

Every hour does not have to be documented notes, meetings, appointments, and schedules will suffice as proof.

Again depending on the sq footage I would guess that a 200A service with distributed to 100A panels will suffice.

@Jared MontBlancThere are several considerations that can go into the analysis of whether you need an LLC or whether a large insurance policy will suffice.

As a Plan B and to follow up on what @Aditya Maini mentioned, I've also calculated rents as if I needed to rent the whole property as one unit to be sure the rent would more than suffice if needed.