I am from NY and it is nearly impossible to do a wholesale directly from the home owner. 

I did a lot of new construction in Charleston SC and did full gut rehab on 2 small MF>  built about 35 homes.. the prices in charleston are very high compared to say Columbus OH. in the order of magnatude to 2X to 4X per prop.. but its a fabulous city. 

My husband works for a commercial developer (W-2) and is a small owner (K1) so our understanding is that he qualifies for REPS.

If they say no, you might consider small claims court.

It was too small and I couldn't help them, but if you can swing one per year that is a cash project I'd say do it. 

I will have to contact him directly.

Any experiences or advice on this would be greatly appreciated.Small Business Loans: Has anyone successfully gotten a small business loan to fund a down payment?

You may have to be okay with just breaking even in the beginning or only getting a small amount of cash flow for the first couple years. 

I've included a preliminary floorplan below.My goal with this ADU is flexibility in terms of renting it out either short-, medium- or long-term rental in the future for income, as well as maximizing resale value (main property is 1300 sq ft 3/2) to someone probably with an investor mindset who would want to house hack.My main concern is whether it's a good idea to have 3 en-suites at the expense of having a small living room/kitchen area.

Best bet is to look up self-directed IRA holders.