Yes, the bottom and top shelves are adjustable.
This product has a distance of 18.20 inches of the flat portion.
even though it has a flat surface on the front the susan shelf is 22". If your inside dimension is also 22" and you are worried, I would go with the same style but in a 20" version
Yes you can adjust the bottom and top shelf to your desired location.
Given those measurements this should work.
The shelves are completely adjustable once attached to the post.
Typically 40lbs is a safe weight. They are rated and tested to hold more, but 40lbs should be plenty for 20" shelf.
The shelves are not adjustable.
You call Rev-A-Shelf customer service at 800-626-1126 and request an email with the PDF of the instructions.