My thought is if you tell a seller "I can list your home for X% and half of that goes to whomever brings a buyer" and they say I wont pay a buyer agent, my response will be "that's fine, I am still charging you the same, but expect your home to sit on the market longer."

I financed half and half of the downpayment with my parents.

Check them all out and see which would be the easiest conversion from your current situation. 

I will honor my agreement of refinancing or doing a loan assumption as soon as I'm able (I've been trying excessively over the past decade), however if he does force a sale, can I tack on the fact that I've paid all expenses including the mortgage for the past now 13 years to demand he reimburse me half, or is he entitled to half the equity and I'm just screwed out of everything I've put into the house?

Here are the reasons:Optimal Financing: By moving into one half of the duplex and owner-occupying it, you can access the best financing options available.

I currently own a condo, a duplex, and my personal home (half of which is rented out) all under my own name. 

You can best cash offers at least half of the time if you are creative with terms. 

The highest bidder was half of my offer.

Some "dude" just saw that other grey walls, white shaker cabinets, and a white-washed style LVP sold down the street and so that is the easiest way to make a buck.  

You're going to have a hard time finding renters that want half a house.