House hacking is such a great strategy for a first property—I've seen family members use it successfully and build wealth in the process.

I offer some of our documents in the BP File Place, available to you for free, look under Other Documents for these.As for the most recent challenge... if you have a clause in your rental agreement about unauthorized occupants, enforce it.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

I live in the Portland, Maine area and am currently launching my real estate wholesaling business as a short term strategy in order to build knowledge, capital, and a network so that I can eventually start investing in my own projects such as fix and flips, and both short and long term rentals.

It would still appreciate overtime and build equity all while someone else is paying the mortgage.

Go to the build your team tab and find one here on BP! 

There's not much room in the yard to build an ADU so I'm unlikely to do that. 

You would have ongoing filing requirements with the State and would need to keep business records and documentation.

Many things go wrong in construction, some of which are unavoidable, how the contractor handles those issues is paramount and speaks volumes to their abilities (or lack there of in the cases above).There are no short cuts to construction or dealing with the building and safety/planning departments in cities, they are slower than turtles in almost all cases.

Older buildings likely have nothing but one layer of sheet rock between the floor boards/joists and lower level units.