But I would think that either a family would take the apartment (husband wife and two children) or college students and single professionals willing to roommate with each other.

Idylwood near East End is a cute little place that you may like, and could probably get decent roommates that are grad, law, or med students (proximity to all schools). 

If he had an unauthorized roommate, is there anything I can do about that?

If 35% to 50% of income vs. rent is not "affordable",  there are a few options,  they could make more money, reduce expenses,  get roommates,  or move.  

What would be a good way to find roommates too have a successful house hack. 

Several years ago, I had tenants who took on a roommate without telling me and they set up a bedroom in the basement next to the boiler.

Renovations to turn non- living space into living space are very expensive, as I mentioned earlier if you are willing to have roommates in a big house you will probably come out ahead. 

I’ll be renting out the upper unit while living in the lower with roommates covering 100% of PITI.

I don't know if that's in your price range but if you added a roommate in your unit and/or did Airbnb it could help reduce that number to something closer to $0.And really, if you found a deal at the 1% rule in our area you'd be looking pretty good. 

I used to rent an apartment where I was dedicated one spot, it drove me insane having to move my car, or wake up my roommate to move their car.