Also signed up for a SelfLender account to build positive history.

One thing you can do is get connected with the local rei’s, go to investor meetings, find the people who are looking for deals and try looking for them yourself and bringing the deals to them since you don’t have the funds yourself, if the deals are good and they sign off on it, it’ll make for good experience and build good relationships from people who can teach you even more, all the while making a little bit of money.

So far I've managed to keep good records and receipts for all transactions relating to the rental. 

I don't think that would be a problem as I have a friend who has syndicated deals and personally signed for the loan.

Having him sign a lien waiver?

It might help realizing too that you're in a market that shows no sign of slowing down.  

Whereas I just wanted a clean financial transaction.

They mentioned two just in our talks, but we'd have them sign a year lease. 

What type of contract or document should we sign?

I used it when I bought my house a few months ago, they put me in contact with an agent and I used him to find/close on my house.I ended up using a different lender (not USAA) and because I signed up for the program at the beginning they still gave me the cash reward (~$1000).