Or they know that thanks to regulators foreclosure now take forever, and that they are going to get to stay in their homes for 1-2 years without making a payment.

There's plenty of post office regulations to google but what works for me is:Line 1 - This can be anything, person or corporate entity, or even blankLine 2 - Must be just above the city and either PO Box or physical street addressLine 3 - City, State, Zip CodeThere are also websites that scrub and correct mailing addresses. 

(We haven't even touched on zoning issues and the growing number of city regulations around STRs.)

There is a Fair Housing regulation that says if you hire a 3rd party to conduct fair housing testing on you, the fact of the testing, and the results, are absolutely privileged.

Fair Housing Regulations is a Federal Law under HUD which is applicable to every State.There may be State specific conditions also, but they can not countermand the Federal's.The gory details of FHR is found at the ECFR Gov Publishing Office site. 

I want to do a shared well (private water supply), and everywhere I look on the Mass DEP regulations, there are no prohibitions regarding shared wells.

It was also brought to my attention during the inspection that the upper unit currently rented only has one entrance/exit and that fha regulation requires 2 entry ways for a second level unit.  

It is virtually impossible to do from overseas although there are some online banks that make the process possible - but if they have no way to confirm your identity (credit check, address confirmation...) federal regulations make it near impossible.

Apparently the gas line on our street is a high pressure and they would to put in a regulator station to cut the pressure down to bring it into our house.  

Each state has their own regulations that  cover the responsibilities of a landlord when a tenant vacates a unit without notice..