*Any advice received is not legally binding as you are not my CPA or Attorney*

Don't do that especially if you do not fully understand the legal trouble, do it on your own.

If the tenant is basically (judgment proof) such as credit is bad, they get paid money under the table as cash,etc. then it can be a waste of time to take them to court to get a judgment.No legal advice given.

You did not include anything for utilities, lawn care, pest management, Legal, accounting, and a number of other miscellaneous expenses.  

There are legal ways to screen for older tenants without discriminating.

I am trying to understand from a legal standpoint, where does a roommate scenario end and an ADU scenario begin?

i understand it was supposed to be done for the price you bought it at ( any legal recourse ?)

There is a 4 unit rental property in Taunton MA that I'm considering but after a little due diligence I found out that it was a legal nonconforming property.

There is a legal concept known as the fraud spectrum that accounts for liabilities that can arise out of our words and actions.

@Serges Lemo,Though I am not an attorney, I am pretty sure any lease extension you create and execute on behalf of the prior landlord/owner, would not be legal.