I give them a deadline to respond, very clear terms of how we will move forward with the option they choose, and I make it clear this is my final offer.

Going to try out PayRange, the Shine Pay is slightly more expensive and the one quote about the 20 units appears to be from shinepay as it is the only qoute they ever made... not sure which is better but both look like good options

To leverage the equity for financing my next deal, a second mortgage seems like a more cost-effective option.

As others have suggested, you could get a conventional or FHA loan if you want lower down payment options.

I’m curious to know what others think about these options.

Give them a couple of options, they already know the unit got foreclosed, most people move to avoid the hassle. 

So those are kind of my options- rent at a loss or sell for the balance.

There are risky, but high return options like QQQ which grew 27% over the last year or on the other side / safe end, there are super secure short term US government bonds like SGOV which pays out about 4.5% in dividends. 

How many mortgage lenders have you spoken with to get qualified and discuss allowable down payment options?

Explore the possibility of doing liability insurance instead, if you don't have much equity it may be your best, and much cheaper option.