I am interested to add a closet, a wall, and a door. 

Also realize to run facebook ads you need to spend hundreds of dollars per day (or even thousands) to et adequate reach. 

Seems like he is only running convenience side of business. 

Often an agent that works with investors can run comps and connect you with a GC. 

So run your own analysis before trusting theirs!

2.If we decide to go with an LLC mortgage ,will three of us have a credit check run again which in case we rather do a conventional and transfer the deed ? 

I do believe it's probably a question of 'when', not 'if', but at the moment, I don't see how that prototype saves any money, considering running wiring & plumbing with solid concrete walls (you'd likely furr over them with conventional stud framing, and would be building 2 walls instead of 1), meeting energy codes in different climates, envelope tightness, and satisfying aesthetic preferences. 

I'd like to next buy a duplex/triplex and step foot through the multi-family investing door while continuing to do single-family house flips.  

I've run some properties about 30 min north of Joshua tree through AirDNA and they show the same occupancy rate as J Tree, 29 Palms, etc. but when I spoke to a realtor about it, he reccomended against Johnson Valley because of how far it was.

That is what will make the difference in the long run.