IMO the better alternative is to build a project that is market rate and includes 10-20% affordable which will allow you to use the State Density Bonus to increase your density by up to 35% (50% in City of San Diego) so for instance a property that allows 10 units can be increased to 15 units . 

In the alternative, cancel the contract.  

You can also see about an alternate payment schedule in exchange for a reduced fee and be sure that they're only collecting if it's occupied. 

Maybe the cashflow from the seller financing will be enough, but if it's not / or he's not willing to accept less than 20% on a down payment, I would love to have an alternative option to present where he keeps a piece of any upside. 

Alternatively, with a HELOC, I could buy smaller properties for cash and then use the BRRR strategy. 

This is probably my largest source of contention at the moment as to whether Id be better off going off with a cheaper duplex with less units, a bit less opportunity, but less repairs vs the alternative. 

I am looking at alternatives financing to Velocity Mortgage?

Though there is some risk of a negative review the only alternative really, is cancelling those listings and asking guests to rebook with the new listing.

My only alternative is hard money and the interest rates and fees are insane.

Alternatively, if you need to buy a home for yourself, there are more and more sellers open to seller financing scenarios too..