So cash on cash is almost infinite.

In your scenario the CoC returns are infinite BUT I would be concerned about how little it is cashflowing and unknown expenses.

Also it doesn't help that you have what appears to be a can of insecticide on the shelf of one of the kitchen cabinets.The Bathrooms - Bathroom 1: That  I agree that  light blue is not a good color choice.

Wood frame with Sheetrock over concrete slab will have a different shelf life than brick over basement construction. 

The only call out I possible have is that your partner is getting a 15% return on their cash and you are getting significant higher return for that, essentially an infinite return on money invested.

I have a feeling that there won't be many off-the-shelf spreadsheets to assist in evaluating this, but maybe I could take an existing worksheet and modify it to fit my needs.

A verbal offer is a "wishy-washy" offer with no real power or shelf life.I had an attorney once tell me "not written, not said".  

Off market opportunities are out there, but as more and more people buy off-the-shelf marketing and data, for instance absentee owner lists and yellow letters and postcards, property owners are being pounded with mail that only seems to serve to desensitize them, leave them numb and unresponsive.Today's successful investors utilize smart marketing strategies to identify off-market opportunities and multiple media (mail, call, knock) to reach and convert prospective sellers. 

My wife has to feel comfortable living there, I try to keep Reynoldsburg on the shelf but she gets down on that area a bit.

@Brian Simmons offers a good suggestion as well - a reverse exchange which while more costly than a straight exchange is infinitely cheaper than stroking that tax check.