What platforms do you like that are good?

Whatever marketing platform you choose, it has to be consistent.

Income limit is $131,775, buyer must put at least $1k of their own funds down, and these only allow primary residences (but do not need to be a 1st-time buyer). 

Like I said before, primary focus is income - appreciation is secondary as we plan to hold this awhile.Thanks in advance!

So, is it a good idea to start with creating a LLC and website to develop a business platform? 

Most banks will also require the short sale home to be listed on MLS and process throw their online platform so doing this under the radar is most unlikely. 

For my primary, they were 5%

If your primary is 600 sqft, I am not certain an 864 sqft additional structure would qualify as an ADU.

My grandparents own (2) properties, their primary residence (paid off) and a rental property on the same street (has a bank loan with substantial equity). 

If I were the lender to "C" buyer, and "C" buyer is purchasing the property as a primary home, FHA is out because of the 90-day flip seasoning, and Conventional is possible, but again, "B" is not yet reflected on title.