A: Yes they amount on the bottom
A: Min. 17.5". Be sure to make sure the doors swing wide enough to allow shelf to roll freely. Also account for any items stored on the shelf that might be protruding sideways.
A: I measured my shelves and there is an extra inch on each side from the inside of the drawer to the outside of the track that it glides on. If measuring from the outside of the drawer, it’s 1/2”. To be safe, I’d have a 19” opening,