Austin was appreciating faster than non-tech cities.

I got more than enough up front as the non-refundable Option fee to cover all expenses and rents and to get a judgment against the Optionee.

And yet every retirement expert tells you to sink every dollar you can afford in to these accounts. 

I'm not sure why not - one of the great parts about real estate investing is that you can build a team around you for aspects you aren't as strong at or simply want to delegate to someone/expert and do the others yourself - look up "Core 4" for example - where you can have an agent, lender, manager, contractor all at once for your investments and can theoretically just let them do all the interactive work

And, the breakup fee is a non-starter.

In many cases, it weakens the lender's position as they did not underwrite the buyer and an owner-occupant tends to pay better than non-owner-occupants.

A special committee was formed last year, and was to hold 6 non-public meetings and then send their finalized recommendations to the commissioners in November.

Simply put the risk of a non-renewal costs more.

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? 

Most nearby cities are the same.Even in the Denver-adjacent cities that do allow non-owner occupied STRs, they vary widely -- some have caps on STRs, some cap the nights rentable as an STR, some cap the number of vacation rentals one owner can have, etc. etc.This is all to say, first know where you can do Airbnb, then start running numbers.