The part that I am still a bit hazy on is regarding the State and City regulations regarding MHP's.  

banking regulations/policies change. 

I do not use any payment software as I didn’t see the need for one rental.For background I used smartmove through bigger pockets.Lease agreement the county property is in has a standard one and I wrote a rules and regulations addendum to it for things like who mows lawn, who to contact in an emergency, no smoking, vaping....

Here, you are offering a security, which is also tightly regulated.

Obviously, MA is famous for being highly regulated, so I don't want to end up with problems down the road. 

However, I'm aware that due to COVID-19 some banks have changed their regulations.

One of them is not moving forward anymore, because there was so much confusion in the planning department there about the new regulations and how are they enforcing them

Are there any laws or regulations on private investing to know about?

A part of me thinks there should be strict regulation in this area where inexperienced investors or those lacking the right tools or are impressionable may fall for questionable marketing tactics.

I'm not sure what the regulations are where the duplex is located, but maybe you can put some pressure on them and say that they either sign a lease or move out.