By law, you are required to send a security deposit return letter to their forwarding address (hopefully you have it).

(I'm in NJ, I can't find any laws on changing locks on abandoned apts, only laws on abandoned property)

What matters more than what the law says is what the courts has already said about the law. 

If your “significant other” insists on getting a house....then buy a house with an in law unit and househack or Airbnb accordingly.

I also found a rare SFH with an in law suite with separate entrance which has allowed me to find some creative methods of continuing my house hacking ways. 

The property has an in-law suite that we currently rent out.

My whole question is do rental income payments have to be sent to a joint bank account and what laws and regulations are there requiring this?

you need to walk into the city planning, smile and play dumb... give them a sob story about your sick Mother in law, and wanting this property, but you just dont know how they handle things. 

You just need to ensure no disqualified parties are involved in the transaction, and that the note is in conformance with state lending laws/disclosures.

Anyone familiar with any reasons/cases/laws of why this would not make sense?