The details are important (like if vacating is part of your deal or required, see below).Each state can vary and each situation turns on its particular facts and circumstances.

So I have tried to read up on laws in the Seattle, WA area and it seems that I am allowed to have a central heating system and just put in the lease that the temperature can't be set to lower than 68 degrees.

The LLC has a lot of benefits (asset protection, anonymity to some degree, etc.), but it also has a lot of drawbacks.  

An identical floor plan & SF, 2 BR condo can vary $50k where I live depending on the upgrades (original or upgraded kitchen, baths, floors?)

Keep in mind, this is just an example and will vary depending on the actual numbers you work with, but carrying the financing and keeping that tenant in there with a lease option would be a great exit.

They also don't state a degree of maturity requirement, so it is conceivable that a puppy would qualify as a service animal under federal guidelines.  

(yes, I know this may vary by city, but I'm looking for basic advice to get started)Looking at a property that is currently listed as a single family home. 

The costs vary depending on the company.

This all depends on the lender and varies. 

If your son has finished his degree recently, he can be eligible with short working history.