Is it possible for a property zoned Residential (R20) that is legally conforming to have a highest and best use as commercial via an appraiser?

First, we don't know what your assignment contract or the purchase contract says.  

If they own several they are obviously investors.Once you get a response from the seller of the vacant house, negotiate a deal to buy it for all cash and get it under contract.

Put it under contract and go to your local investors club.

So you're saying if I move my mom into the other unit and occupy one myself, the vacant can be vacated legally?  

The attorney won't walk you step by step through the process like an agent would but the contract you sign with the FSBO can be structured the way you want it with an attorney.

I sandwich lease several SFH but there is a very big difference in the contract with my seller and the one with my buyer.First of all my seller(owner) is well aware that I have no intention of living in the home.

I also work part time as a personal fitness trainer and I do some small renovation contracting work on the side for family and friends.  

Makes sure you add a clause that says its contingent on close of escrow Ect. because otherwise it's a breach of contract.

If they own several they are obviously investors.Once you get a response from the seller of the vacant house, negotiate a deal to buy it for all cash and get it under contract.