I've always only included interest portion as the mortgage expense since the principal portion is equity/planned savings and only increases my cash on cash ROI over time. 

I want to use the BRRRR method but am unsure what loan options would be the best for the renovation portion of the project.

You should be able to deduct some portion of your home's expenses.

There was a little wall/dropoff where the leveling compund ended and the original floor remained so i had to use floor mortar to feather it down....still when walking on the new LVP, i can tell where the dropoff is/was.SO, next time i will 1. make the SLC more runny, 2. on a small job (500sf) ill plan to cover the entire space with the SLC so there are no transitions, and 3. the LVP with the underlayment attached seemed to work great on the portion of the floor that was on slab, but feels cheap to walk on in other areas - so next time i will most likely go with a product that allows a thick underlayment to go own underneath it.The last thing i learned was that the 1/4" gap around the borders that is recommedned is too generous as it is almost impossible to hit on the nose. 

Say for example the entire shared portion was on the other neighbors property.  

The property had a large double lot in a nice neighborhood, so in 2014 I subdivided the lot and sold the empty portion of the lot to a builder, and kept the main house on the lot that remained. 

I would be living in the bottom portion of the two homes for a little while with the upstairs 2Bd/1Ba unit being rented at $450 and the back home at 2Bd/1Ba being rented at $550.

By doing this you can ultimately get a portion of the profit tax free although you will have to re-capture depreciation.3.

He is saying that he can not take any portion of his house and start depreciate it?

But you may owe a good portion of that back to the government after selling.