Working with a licensed agent at least has a recourse via his license and the board of real estate.To counter those issues, I would need documents to show me "I CAN TRUST YOU".

I spent some time on it and created a workflow that automatically creates an excel document for allowing me to book my mileage.All I have to do his hit a button on my homescreen, enter my mileage from the previous trip, pick between personal or rental and it adds a new line to  the excel file.  

Also, what documents would I need to produce in order to apply for the mortgage?

I say "might", because I haven't actually seen an Oklahoma court opinion on this issue.In my opinion, the safest way to document this transaction is to execute a standard lease agreement like you would for any other lease transaction.

Do you by chance have a prepared document outlining the actionable steps to going from say 'poor credit,' to getting a mortgage?

Does email work for that though, since it's at least documentation?

Except that you have told her that she can stay.If you want to give an incentive, I would document a few options on paper, go talk to her, and leave the paper with them to consider their options.Is the house likely to remain a rental, be owner occupied, or could go either way?  

This theoretically triggers a default clause on the loan document, but as the mortgage will stay current, lets assume nothing happens.5.

Would the loan be possible without any other documentation?  

If you can are comfortable with the other documentation, and they can afford the rent based on their income, and you're comfortable with their credit worthiness, what's the problem?