After we signed for the mortgage, he decided not to move in even though the conditions of the mortgage required him to do so.

Unless the land had such a signifacant value like over $500K you might find a private lender to put a junior lien against it if you own it "free and clear".I have seen some private lenders offer a higher LTV even 100% if the collateral is sufficient, but again land is tough to resell unless its commerically zoned in a great location.  

Townhomes are a great start because they often have lower maintenance costs and better cash flow than single-family homes.

Our housing stock is mostly single fam. 

This just might be my opinion but at this point even if you're out one months rent which definitely sucks it may just be easier to start showings.I don't know what the L/T laws are in your county/state but perhaps send a notice to the emergency contact with something to the effect of pay or quit with an option for a release out of the lease and be done with it. turn over this unit and get it out on the market again.

We are more specifically interested in single family and multi-family rentals.

In other words, how would I calculate the return (from a purely financial standpoint) of "investing" in a primary residence vs renting and putting the money elsewhere (perhaps even into an investment property, but also considering buying a business)? 

We currently have a single family unit that used to belong to my wife’s grandparents that we turned into a rental. 

Also remember that Real estate is a long-term play so even if the cash flow isn't a "home run" right now you can refi later and hopefully even pull out some equity you can put down on another one, bringing you closer to financial freedom.Thanks for reading BP!  

Even with nonconforming, can it truly be a comfortable bedroom?