However, you can't sell something unstable, with circular references, requiring human override, etc.But, if you stabilized it, the model loses efficiency. 

If you meet that requirement, I highly recommend it. 

That is often a matter of opinion and will depend on the attitude of your municipality.Most municipalities do not require neighborhood approval, but will offer the public opportunities to provide input which will be considered.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

The fix/flip might be a good way to get such a relationship started: usually has lower cash out of pocket requirements and you can sell the property faster to recoup the capital invested, plus any profits.

Make sure to get legal advice on what you can and can’t do for bill back and what kind of notice period is required in your state. 

To vet guests, you can use a pre-booking form, ask about their plans, and require an ID and signed agreement.

They/you can't choose the carrier - their software assigns it.Fortegra is one of the carriers (paper in insurance parlance) that their product is written on.That said, Fortegra is A- rated I believe by AM Best so that isn't the issue.I'm going to take a guess and say that your umbrella is written with PersonalUmbrella.com (Markel) and they require all underlying insurance to be A-rated but also with an admitted carrier. 

All the leads I've found so far have balked at either the "investment property" requirement, the "in California" requirement (or both!)

Then add any other codes required where you want to build and study those as well.