They may have a challenging time finding a buyer with a lender that can finance this property. 

I prefer public comments here before you send a direct message, though. 

It is a older non rehab property but things are straight and roof, plumbing, electrical are all in good working order.

DSCR lenders can cash out up to 75% with little or no title seasoning requirements. 

The other challenge is selling it, there is a smaller buyer pool for hospitality in a small market but that was known going in Did you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?

Non pay of prop tax's non pay of insurance  Waste, etc. 

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? 

You got some solid advice above check with your lender and get in contact with an attorney like @Bob Floss II who can help setup the OA.Assuming there is no pushback from your lender, you should be able to do a quit claim deed to retitle. 

For reserves, some lenders want 6+ months. 

Gifting equity in a house may be more trouble than it is worth if you are not near the life-time exclusion1) Potentially having to get the property appraised everytime an equity is gifted2) Having a title company update the title every time3) Exemption of the gift tax return is normally for cash gifts below the annual gift tax exclusion, you may still need to file if you gift non-cash4) If there is partial ownership over several years, having to properly pro-rate certain items such as real estate taxes, mortgage interest and rental income if the property is a rentalBest of luck