A collection agency, PI, or a company looking to perform due diligence on its customers would have permissible purpose.  

Lots to the story, but it boils down to poor work being performed initially and a project manager that let things slide.

The market and rent appreciation really made the properties sing and perform phenomenally.  

I'm trying to learn whether an exit on a property should be dictated by a time frame (5-7 years), a property's performance, or a combination of the two.  

Do any of the services above need me to be a real estate broker to perform on behalf of another landlord if I charge them a fee to do this?

And if the OP doesn't want to perform what most, frankly, would consider a small act towards developing a relationship... why would the bank want (using the OP's language) to go out on a second date?

My health took a turn for the worse, and my performance on the job suffered greatly because of the treatment.  

As I see it, your options are a warning letter/discussion, a 3-day to perform or quit (regarding quiet enjoyment of premises), a 3-day to pay when the rent is due (but I stress again constantly collecting on the 10th has tacitly made the due date the 10th unless you reset with a change of terms), or flat out termination with a notice to quit (30 days if living there for under 2 years, 60 day for 2+ years, or 90 day for a HUD tenant or as your housing authority requires).

Chances of his bill equaling exactly what is due from him are virtually guaranteed regardless of work performed.

If your agent has given you a modification (or addendum) to the contract asking for your acceptance with the work already performed, then your refusal to accept the modified contract without any counteroffer should close the negotiation and void the entire agreement.