If you put less than 33% down or had at least a little cash flow, you have even more than 100% ROI.I would say that's awesome.Naturally, you never know if past experiences will be the same in the future.If I had/owned that property I would get a HELOC or all-in-one loan.

Please clarify this.Clarification: 1) it's a discussion.2) don't assume all realtors or all people are represented by something you've seen in this, or other discussions. 3) it's a multifaceted world in which we can have the humanity to consider our neighbors as well as our investments, and that is a worthy topic of discussion.  

Payments that you make normally fall into one of 3 buckets100% of the payment can be factored in somewhere on the returnPartial payment can be factored somewhere on the return0% of the payment can be factored in somewhere on the returnHouse-hacking also has considerable tax implications in the event that you want to sell this property.You can potentially defer a portion or all of the gain on the investment property with 1031 exclusion.You can potentially exclude a portion or all of the gain on the personal residence with section 121 exclusion

If they just want the highest sales prices, and aren’t in a hurry to sell, then maybe they don’t care if it’s a financed offer or all cash.Realistically, once the deal closes, the seller is getting paid the same (all cash directly to their bank account) either way.  

He has some mental issues, but had lived in the home for 15 years and acted as landlord for some or all of it.

You've gotten really good advice here so I won't duplicate any of that but agree with most or all of it.If it were me, I would hound that caseworker like a crazy person. 

many tenants  are  not paying  any or  all or their   rent in KING county 

(Or all leases are utilities included?)

Or, they can be the sole mortgagee and you can just be on title with them and maybe gift some or all of the downpayment.

In this case, they they want to gift you a portion or all of one property they might want to hold/straddle for at least three income tax returns to help demonstrate their intent.