I wouldn't attempt advising very indepth on this but have a couple of thoughts. You must contract this to somone you trust then follow their advise. If you don't have a knowledgeable friend to assist you ,inquire with the closest concrete plant for referals of contractors.
Unless you are attempting to have the patio and house floor level with oneanother,I see no reason for doweling them togeather. If you want them level,doweling alone is worthless and requires desighn by a pro. I am curious why you are repalcing the patio in the first place.

Couple of things....

A deep pour (12") is very heavy, and prone to settle and crack (even with mesh or rebar embedded).

I'd say get a good layer of class 5 limestone, compacted for a base, and pour a normal slab. You can't tie the patio to the house, the house has a frost footing.

I must be missing something here. If the slab is poured on ground how can critters get under it? Underpinning might help prevent heaving of the slab but this is generally done to build an addition as is very technical and expensive.