Bought it as a safe bet thinking it would do roughly $35k annually in short term rents.

Every unit has its own "microsite" of sorts with seperat msg threads and you can't write your own descriptions (which is understandable as they don't have seperate websites for different countries so they write "translation safe" descriptions for you), but still, it's no Airbnb.

I really see Columbus Ohio as an extremely safe bet for the next 10-20 years.

its highly risky for most mom and pops to sell on sub to.. and its simply not a way for those without substantial wherewithal to buy property and keep the seller safe..

That is a safe harbor from the IRS with a couple of caveats. 

If we bite off people’s heads when they admit disaster, it suppresses legitimate issues with these techniques. 

I know that the general guidance is that emergency funds aren't to be invested, but given our financial security and the fact that we already have a personal e-fund in a safe, non-volatile location, would it still be crazy/unwise to invest this 15k in an S&P500 index fund in a brokerage? 

It is a nice safe long-term spot if you're approaching it with a more long-term traditional real estate mindset.  

I likely would take a safe approach in my underwriting and value it with the quads then subtract 15% off the total.  

These same banking regulators required banks (which include Trust Companies) to hold reserves of safe assets.