Are the going/common down payment minimum for duplex's (20%) tri/quadplex (25%)?

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. 

A lot of them contain Balloon Payment Provisions. 

When do you make owner payments?

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

Total interest paid drops to $43,031.89 a savings of $34,633.44 in interest paid to the lender.If the lump-sum payments come out of our savings, then we are smiling foolishly as we have saved 44.6% of the originally scheduled interest.Things are a little different if the initial lump-sum payment is borrowed from a secured LoC - which, for the sake of simplicity we will assume to also be at 3% compounded monthly.  

If you have that experience, you probably don't need much marketing, as sellers will be reaching out to you directly.Focus on securing one listing at a time and building momentum.

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? 

do not use a line of credit for a down payment. 

Assuming plans are needed to secure a permit.