What if a major employer in the Portland area shut down causing a regional issue?

Same as aboveYour views are appreciated.I will note that if you are having one broker put it on the MLS and another one perform tasks such as advising you and Open houses, you may cause legal issues depending on the wording of each contract.

I see no circumstance that would cause me to sell a cash-flowing property, or in this case, cash-flowing THOW. 

Examples are: tenants who party, sell drugs, make too much noise at night, have police called on them regularly, cause problems and generally depress and bully other "good" tenantsSome reasons I can think are:the tenant is on disability and the rent is guaranteedthe tenant is somehow related to the landlordthe landlord is negligentAre there any other other reasons why nuisance tenants are not kicked out?

They will often cause much more trouble knowing you're the owner and can change anything you want about the property.

Initially, I too wondered if the lack of a driveway might cause delays in finding tenants.While some applicants have mentioned they wished the houses had driveways, I can only think of 1 or 2 in 5+ years who have said that was a factor in not pursuing the rental. 

It is reasonable to negotiate based on the findings, however, if you have little to no experience on what the issues cost to fix, you could really be causing issues.

This gives the tube more surface area to transfer heat but it also causes portions of the tubing to become thinner.

I don’t know that I have cause for one.

Doesn't seem like a big cause for concern right now.