I went so far as to buy a dehumidifier so he could use the basement for storage, replace the screen his dog damaged at no charge and other little things I don’t technically have to do to make it nicer for him  Now I feel like he’s taking advantage.Now the reason I’m considering giving in. 

But if this is the last affordable complex in the area and the price increase to "market rates" causes them to move much farther away and uproot any connections (like daycare and the like) that they have in the area... well, I'm not a fan of that.Moving on to self storage.

The 1970's attic apartment was just wasted storage for 12 years so we rehabbed it for $2500 & our live-in maintenance guy now pays $475/month plus does any & all building & property maintenance.But we do all the work ourselves.

The feasibility of which is highly dependant on your decisions on the previous steps.In parting, there is an important concept that you will need to learn about investment property loans. 

I have read that for determining equilibrium a good rule of thumb is to use an avg of somewhere between 7-8 sq ft of self storage per person within the radius you are analyzing.  

How feasible is that?

There is also a large 12x14 storage room that could be converted into a room too (or opened up for a kitchen).

We are implementing a similar strategy at 18 units and since have put a multi and a storage facility under contract with 3 more offers out.

Would it be a good idea to put in mobile homes (assuming that we could get approval from the township), put in storage spaces that could be rented out or to build additional rental/condo units? 

You can diversify within real estate itself, be it acting as a private lender, purchasing notes, investing in a fund, buying a storage facility, etc.