He has not made it despite living with his parents most of his adult life and living off of them for tuition and other expenses too. 

You're playing the adult real life version. 

The house is a multi-use home because there is an adult family home business using the house.  

My husband and I each work full time W-2 jobs, and the plan is that two of our adult sons will do a majority of the rehab work- but they can't start until June so we're just spinning our wheels. 

If you are confident in your real estate knowledge, market knowledge, understanding a sellers options, creating a CMA, calculating repair estimates, identifying motivations, negotiating techniques, marketing capabilities, client accountability, ability to manage highly stressed adults who are selling their LARGEST ASSET, by all means, try whatever works.

I am a young adult and I have been pondering on real estate for quite some time.

Here's another opinion:  When you say 'child', do you mean adult child.  

I can't call until Monday, so I figured I'd try here for answers.How do I calculate the "number of adults living in the unit"?

I mentored my adult children to do the same.  

What paint (primer and finish) would you recommend for a 300 sq ft in-law unit which would be used as a rental for 1-2 adults max.