Looks like they invest their clients' money entirely in ETFs as opposed to mutual funds, so there's less fees because of that and their ability to serve more people online. 

Sounds like if Zillow can crack the nut of interfacing accurately with all the different systems, and if there can be a fair price paid by Zillow for access to the MLS data, it could end up being mutually beneficial for all parties. 

However, my inlaws basically want to sell us the land in a way that is mutually beneficial.

Again, if we are only considering the affect on credit scores, and with all the caveats about speaking to the experts in your specific area:  If your business is new, it may be beneficial to go that route and establish a business credit profile for future business transactions. 

Are there strategies that would be mutually beneficial to both sides other than a straight purchase and sale?

Ive been with liberty mutual for my home insurance and umbrella policies for rentals.

To make things more interesting, yesterday was the last day he had to send a mutual release and he hasn’t done so yet.

I'm thinking this would be beneficial as I wouldn't have to worry about the tenant not paying rent, I would be building equity over time in a market it would otherwise be very difficult for me to enter, and I'd be able to minimize fees by finding the lowest cost property management possible (as I wouldn't need to worry about tenant screening, etc). 

This week I am meeting with one of the top loan modification experts in the country (through a mutual acquaintance). 

Working with an agent is a mutual relationship.