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? 

Again, knowing nothing about this particular company, my bet is that type of company will require money up front just to consider whether they can help you out.  

Safe rent after rehab is $900 per month, maybe higher if I get lucky and/or rehab it to a very nice condition.  

Hahaha I'd bet anyone under 40 doesn't even know what that is :D OK.

That’s the best bet.

It’ll save you headaches later if she tries to escalate things.Document Everything: Take pictures, write down the history of your interactions etcOffer a Transition Plan: Just like my client, I’d recomend offering her a temporary storage option nearby, or even a small rent discount, to show you’re not trying to kick her out - just keeping the property safe and up to code.

I’ve already put down a $2,000 earnest deposit and paid for the inspection and appraisal.This event is unprecedented in the area, as the area is typically low in violence and considered very safe.

Your best bet would be to connect with local operators and ask them.

There really isn't a safe, secure way for someone to protect their interests.

I bet there were some takeaways looking back.