Right, Lix will always show an end-of-level screen, whether you escape, nuke, or win. This is for consistency: To escape, you can always press Escape twice, regardless of whether you've won.
After quitting the level, I can offer the next levels or can leave the bottom of the end-of-level screen blank. The code will be simplest when I always offer the next levels, but maybe that will look strange? Or we can have a first version that always offers it, then decide later.
-- Simon