Her apartment can easily be converted to an open concept 2BR. 

I've also come to the conclusion that you are either 1 bedroom or 5+ bedroom. 2-4 bedrooms are the squishy saturated middle. 1 bedrooms make good income relative to purchase price, but a 1 bedroom SFR (assuming you can find one) will get killed on the recurring expenses that don't scale down like WIFI/Cable, STR insurance, lawn care etc.

I understand the concept of doing a 11031 into a tenant in common situation. 

There are no shortcuts really because it takes time to grasp a lot of these concepts.

The concept is good but do the numbers work?  

We are not talking about the larger concept. 

Good question, and this leads to the concept of a net tangible benefit.  

First step is determining the viability of your concept. 

Even in a less invasive county, I’d start by scheduling a pre-application or “concept” meeting with the planning or permitting department before doing any work.

The same concept goes with neighbors.Just something to consider.