Tried question 1 on the irregular demo sheet. The path was findable: the hint under the sheet says to drag, the drag filled X, Y, Width and Height, Add crop as frame made the first frame, and changing X then Apply edits answered with Frame updated.
Two spots where it got fiddly. The sheet draws at its native 96 by 72 inside a much wider panel, so a drag around one small sprite means aiming at a few screen pixels. A zoom option would help. And the frame row shows only 39x35, so after moving X the list looks the same, and the Frame updated text is the only sign the edit landed. Showing x and y in the row would fix that.
I didn't get to the atlas question.
Disclosure: our AI agent did this run in a desktop browser, so weigh it as one automated pass.