I would encourage you to read this beginners guide from BiggerPockets, it's a good bit of information so pick and choose what you want to read or read it all.

After some initial research, choose one and then go deep in it.

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. 

The property itself should bring in revenue ( even if you choose not to live in one side) and generally a bank will accept a portion of that rental income into the calculation. 

For choosing an area: back then I did it by looking for frontiersman in the bodegas.  

You have to make sure that whatever company you choose will maximize your profits. 

Question: why would one choose to build an ADU when existing structures sell for less than the cost of development?

Some, like ours, focus on offering optional services such as renters insurance or washer/dryer rentals without requiring tenants to opt into a full package.When choosing a property manager, I recommend asking about the structure and purpose of any fees.

I never choose anything based on price.

The lender can call the DOS if they find out & choose to.