I've never heard the 25% thing before, but any difference between FMV and sales price would technically be considered a "gift" with potential tax implications for your parents.You can avoid most seller costs as there's no reason to use agents in this case.As an FYI: I always recommend avoiding a FHA loan, if you qualify for a low-down payment conventional loan.

Trailers don't go up in value like conventional houses.In my opinion to get the true benefits of buying mobile homes you want to own the land underneath them.

If you go with a conventional loan for the cash out refinance, seasoning periods are longer which draws out the process. 

In my experience, you are thinking through this correctly.I've heard lawyers say, if the loan is in your name (which it has to be for conventional) and the deed is in the LLC, you aren't treating the business as an independent entity and in a severe lawsuit, the corporate veil is pierced and the LLC can be thrown out.I agree that one of the best defenses is to run your properties well. 

STR standard depreciation is 39 years @Andy Lanyi@Jonathan Cooper even if you take Bonus Depreciation as @Basit Siddiqi highlighted the remainder 40% will be prorated using the Half-year convention or Mid-quarter convention used in line with what you read in IRS Pub 946 

@Rick Albert   I may get a lot of push back from the team but I split my utilities on my ADU the conventional way....each dwelling has its own meter for water, electric and gas. 

What happens if you buy the duplex with a conventional loan with the purpose to build an ADU to move in shortly afterwards?

Quote from @Kevin Prasad: Hi all, I’m looking to negotiate a deal thats a mix of seller financing and conventional lending.

About 20% use 15% or 20% conventional (my lender does have a portfolio 15% product currently). 

Also, make sure there aren't additional restrictions on the property condition, compared to a conventional loan.