Newer roof (2019) and newer water heaters and HVAC.

We could sell that house by itself, but it would be sold under a land in common agreement and an HOA would need to be formed.I also confirmed with the Public Works department that a 2nd driveway can go in, with a septic designer that there's room for second septic tank, and with the public water company that they can tap into the main water line from one of the neighboring roads.I'm sure there are some more details that I would need to iron out that the code officer didn't bring up, but the high level answer I was looking for is that my town does allow selling off a second house under the LD 2003 legislation.

We put in around $15,000/$20k worth of construction & furnishings to bring it up to Airbnb standards.

If after adding the additional monthly recurring costs (electricity, gas, water, internet, insurance, lawn care, snow plowing, etc.) to the standard long-term costs (Mortgage, taxes, regular insurance, etc.) the property won't net at least $100 if I can get the average rent I see on FF for that area, then the property isn't worth more investigation.

. - unless you get some whacko judge, there should not be any issue collecting rent on unpermittted unit, having a move-out cost (no fault eviction) in excess of the standard rent control or eviction process in excess of standard rent control hurdles.    

I've been introduced to GDL Asset Management/Green Water Asset Management, which primarily focuses on multifamily properties in Arizona.

The 2 main issues are that there's a leak in one of the bathrooms that is causing water to flow under the sub-floor and into the crawlspace. 

One tenant in particular that was a nightmare (we had to evict and learned some HARD lessons with house standards), which impacted another tenant who started also being difficult. 

If you are taking extensive time managing, spending money on gas, repairs and maintenance you are under water.

If there are excess funds after paying the local/school real estate tax bill, the remainder can go to:Current real estate taxesCurrent Muncipal liens for water/sewer or any other bill initiated by the municipalityState Liens, usually for outstanding state taxes, sometimes employee/employer taxesPA State Inheritance Tax which applies to all deceased owners and starts at dollar one with no exemptions, unlike the Federal Estate Tax.