Long story short he cheated on his GF who basically paid everything now he will not pay or move.

Former would be permanent while the latter could be temporary (as long as the barn or fences exist, for example) if desired.The surveyor you just hired can help with the language required for either. 

i am looking to invest in a long term multiple family near downtown.. the prices are much better than north jersey ( where i live) , but I'm not sure about the rental markets and typical cap rate. let me know if you're open to discussing further

You'll want to ensure that the lease clearly outlines the terms and responsibilities, including occupancy limits and any necessary modifications to the property.

Without more context, it's impossible to know whether the owner financing terms are reasonable or "ridiculous." 

If it’s feasible, how long might the process take, and what costs could I expect for the necessary permits?

As long as the borrower is current with taxes and insurance and continues to live in the home as their primary residence the lender cannot call the loan due.

Owning a house together is like a long-term business partnership, so make sure you have a clear exit strategy in writing.

I would do that all day long, personally.  

She has had a year long lease then another year long lease renewal addendum with the old landlord.