However, I find myself uncertain about how to structure such deals and whether I could successfully secure them.I was wondering if you could offer any recommendations for educational resources that might help me better understand and navigate the intricacies of creative finance and seller finance deals.

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? 

Focusing on the best quality real estate will allow you secure more favorable financing terms, more likely to achieve favorable exits/refinances, more likely to attract the best capital partners.

With the future being so uncertain I realized there was a need to do more than just rely on my job for financial security. 

That doesn't seem right about having to chase your money - but would need a lot more info to provide a better opinion. also are you investing in one property and is your name secured or you giving an unsecured loan? 

Begin by targeting distressed properties in gentrifying neighborhoods, securing financing through hard money lenders or private investors, and focusing on cost-effective renovations.

Separate accounts per property incurs a lot of overhead, such as minimum balances or fees for missing the minimum.I use separate accounts for each tenant's security deposits.

Loan Security: The property will serve as collateral for the seller-financed loan.

Unless the medical condition meets the state criteria for early termination and that the medical notice does not appear to suggest that, you cannot terminate the lease outright/immediately but that you will begin to market the property for rental and upon securing an acceptable tenant, the lease can be terminated pending a satisfactory property walkthrough indicating no damages. 

If a single-family owner-occupied property is worth $400,000 - I wouldn't invest more than $280,000 (70%) in a note secured by the property.ITV takes priority over desired yield - so even if the note discounts to $350,000 based on desired yield, the investor still won't go beyond their maximum ITV.