Buyer will miss on deal after deal..

I have been using the tools learning tools BiggerPockets provides to find deals and analyze deals. 

Most important is to find the best deal possible, if you find a great deal there are always partners for that project.2.

My parents who invest, say they never do deals with more than 15 year amortization.

I am currently considering a deal on a very small park as a way to test if MHP's are going to be for me or not.

They should be able to guide you through the benefits of each scenario... just tell them you're thinking about living in the quad or potentially just renting out all four units.

Sounds like you need to sell one or two, or you can do a HELOC (check more than one lender on HELOCs on them) to value add the deals (ADU, and or buy another property to cashflow). 

About to close a deal all cash for $1.46M and was going to get a cash out refi after to take out about $630K in cash.

He "doesn't want to scare off his tenants if the deal falls through." 

However if are dead set on Owner Financing, you have to get get 20% plus down payment and if you are in Florida, you have to deal with a Judicial foreclosure process which is not nice to Lenders, I would highly recommend against it.