Help with deck footings
When I did my deck, and did my design using a CAD program I have. I carefully measured existing elevations, and fixed the elevations of all the beams. The footings were no problem, because I used 6x6 posts attached to the footings using galvanized brackets (I used the Simpson model, but there are other companies that make similar brackets). I cut the posts at the correct elevation after they were installed using a handsaw.
If you plan to use short posts to support the bottom of the deck stringer, you can do something similar, assuming your building inspector approves. The wood posts are supported on brackets, so there is never any contact between the PT posts and soil.
As for using sonotubes (cardboard forms) versus just placing the concrete in the hole, you can do either, but you save a little concrete with the sonotube, and you prevent the hole from collapsing and getting soil into the concrete, which weakens the concrete. You also get a neat appearance for the part of the concrete footing which sticks up above grade, which in my case was about 6 - 12 inches.