Does the area (1256 sqft) need to be one open area or can be split into multiple rooms with doors open ?

Well first off no real air purifier expert would tell you to use the 1256 sq.ft. You want a minimum of 5 air changes per hour (ACH) and 1256 only provides about 2. If you are truly looking for the benefits of air purification then you should be looking at 500-700 sq.ft and no more. With that said, you lose efficiency if it has to go to multiple rooms and thus less ACH. Unless you have an industrial Air purifier built into your HVAC system at your house you should think about limiting one air purifier per room (with the appropriate amount of power to handle it).

I'm getting a shipping error for California. Why is there a restriction to ship to California? It doesn't go over the dimension or weight max.

California bans electrostatic air filters because they produce unsafe amounts of ozone. Ozone damages your lungs. This also might be an Amazon problem, where they take reviews for similar products and stick them all together. I'm not sure the Airmega 400 has an electrostatic system in it. I don't see the no shipping alert on its Amazon listing (I live in California) but I see it on Healthmate HM-400 listing. The Wirecutter did a whole article and missed the electrostatic filter, so thanks CA. Electrostatic filters are very effective at removing particles but they produce ozone. If you're tired of smelling chicken poop, moving from a chicken ranch to an oil refinery would work, but.....

Does this work well to remove pet odors? It is so hard to hard to find any straight forward answers about odor removal on any of these higher end unit

I love it. I have a very sensitive nose and breathing. Works great for pet odors! I need one for the other side of the house. Worth the investment. I've tried some of the other $200-$300 purifiers

Will this work for oil painters - i need it to help with the removal of vocs such as turpentine and mineral spirits?

NOT good at all. I wish I could say otherwise since I own this product. I have 3 different instruments that measure VOCs. DO NOT depend on this product for VOC removal. I bought the Conway and the VOC testing instruments because I am a victim of VOC poisoning in my living space. After zeroing the testing instruments and getting baseline VOC measurements, the Conway did nothing to reduce them - even at the highest fan setting after 24 hours. I still love the Conway though as it does a SPECTACULAR job at reducing a full range of microparticulate matter.