I'm in Northwest Arkansas, and I don't know the Conway market, but with the two colleges there, I bet it's a good place to get started.

I would say your best bet is to find one in which you simply own the land and fixtures. 

There is no one to blame here, but there are some very big lessons to learn and I’m betting the tuition is gonna be expensive.

You can add/subtract a little for those type of things but even with no adjustments, it's going to get you a ballpark number.

It appears to me that, in inflation adjusted dollars, your return will be the current monthly cash flow combined with the equity pay down (this increases with time).  

Since I'm not quite able to buy with cash, I'm curious if any of you have recommendations for local credit unions/smaller banks that offer financing (fixed rate or adjustable) for less than 20% down.

I don't think you'd be able to use comps to qualify but I bet if you talk with a couple lenders, you'll be able to get it financed!

I bet it was challenging at first to move from your SFH into one of these units but in the long run it'll pay off.

And the HELOC is interest only for a period of time before adjusting to an amortizing loan.

Your best bet may be to explore building an ADU on your existing properties, or to buy a SFR and build 1-2 ADU's.