There is risk in both options & as an investor you need to weigh those risks & stand firm on whatever you decide. 

Several options you can explore to get the down payment capital: Solo401k; HELOC on personal residence or from equity of the SFR; Cash Value Life Insurance Policy; Business Credit; Private Lenders, to name a few.

When I help buyers in COS and they see $700+ HOA and I explain the land lease scenario they don't consider it an option.   

Be sure to understand the financing options out there for mobile home buyers and if you opt to carry the contracts on the homes you sell you should do some research on Dodd-Frank regulations. 

We looked at a couple different options as far as % downpayment and I figured I would refinance in the near future to help the payment get down lower.

Even though your market might prove expensive, I would exhaust all options before ruling it out.Truth is, you have a unique competitive advantage in your market.  

Seems like a good option is STR during peak season and MTR in other times of year.

I'm just starting out learning real estate, but I can't decide whether renting or flipping is a better option.

That is the best option if you are truly trying to help a family member.you cannot get control of the mortgage, you can do sub2 but you are not in control, the bank could call it due if they see the property transfers title.The other way is for you to buy it from them at market rate, then rent it back to them at market rate.

If the rent stabilizes, I can then consider refinancing these properties using DSCR or other valid options (please educate me, and let me know what other options you think are valid).