Just have him deed the property to you, no money changes hands, you start making the mortgage payments and get the house listed for sale.

He has a handful of multifamily rentals in my area and I would love to buy them all so it’s important to me that this is an amicable experience and we both get a fair deal in order to work together on the other properties. 

Can a lender "take back" a loan once it's closed and the deed is transferred?

You should have just had the money transferred or paypalled into a normal account

Sounds like you want him out now and money motivates a lot of people.Make it contingent on the condo being in good condition and professionally cleaned so that the expectation is set before handing him over any money.

I think you primarily need to look at the capital you have on hand and define your goals.

How do I transfer liability back to the neighbor for letting the new owner know?

Put comps and a list of improvements in his/her hand. 

I suppose one could say the same about the TK business but I haven't been on that side of the fence so I shouldn't speculate.Speaking only for myself, our investors and our track record are our highest priorities, and these two things go hand in hand. 

I understand how the plan administrator would not allow such transactions but having a Checkbook SDIRA it seems like the plan administrator is very very hands off.