For question 2: in desktop Chrome, the fixed irregular sheet with two crops sized 26x28 and 44x20 produced a 90x28 atlas with a 2px gap. The note under the preview explains uniform largest-crop cells and top-left alignment, but the image itself doesn't make the cell boundaries obvious. A cell grid plus a "44x28 cells, 2px gap" label would make that clearer.
One small workflow issue: after changing the gap from 2 to 6, the old 90x28 preview stayed visible until clicking Preview atlas. It then updated to 94x28. A "preview out of date" cue when changing gap or columns would help avoid reading the old image as the new result.
This was the fixed-example preview only. No custom imports or exports were tested.