Request any outstanding liens, fines,   bills or ongoing litigation on the property.  

Here are a few of the Massachusetts laws, rules, and regulations, and a few Federal ones your property manager should be aware of, and understand to avoid possible problems between tenants, municipalities, and lawyers (not that they are bad people at all):Tenant/Landlord Law MGL 186 ss15B; 93a; 139 ss19Housing Discrimination Law: Title VIII of Civil Rights Amendment of 1968Information Protection Act: 201 CMR 17.00 & 93HState Sanitary Code 780 CMRState Building Code 105 CMR 410International Building Code (IBC)International Energy Code (IBC energy Code)American Disabilities ActGood starting points:Offense: https://www.mass.gov/info-details/massachusetts-law-about-landlord-and-tenantDefense: https://www.mass.gov/guides/the-attorney-generals-guide-to-landlord-and-tenant-rightsSome light reading to keep you out of court rooms...ENJOY!!!

The neighbor's biggest obsession seems to be the retaining wall, which he owns per a complaint he tried to make in 2015 that I saw in the code enforcement website, he had a survey done and the findings were that he owns the wall.The building inspector called me on Monday 7/31 and said he'd give me more time for the other violations if I re-route that downspout as my next project-stated-they've been getting a lot of contact from my neighbor and he's trying to help me avoid litigation.

The problem is that litigation is lengthy and expensive (especially in California).

I mean technically since they forced you to sign that lease and live there against your will that is a crime of false imprisonment and not a civil matter to sue over, but I'm certain that they would be willing to settle out of court to avoid being indicted in federal court. 

On second thought, you might want to just sign off on this tenant, so there's no pending litigation against the property, which could delay a sale.

That is why 95% of Federal Civil Cases are settled.And what and how much are they going to sue for?

Assuming the rental is in seattle as OP is,   Seattle has an (annoying but its the law)  ordinance https://www.seattle.gov/civilrights/civil-rights/fair-housing/first-in-time dictating how rental applications must be processed -  This is the first-in-time ordinance.   

After realizing their lease was expired for a while now, I recently reached out to them to have a civil discussion on the next steps. 

There are so many benefits to having broken down bids from helping you build your knowledge of repair costs, to comparison shopping, to documentation in the even of litigation.