The data is hidden in the image data itself, it seems(!) Does this mean that an accurate screenshot can be used to clone the cartridge? For fun, of course.

Yes all the data (source, sprites, sound) is encoded in the PNG. You can simply copy the PNG to clone it (screenshot won't work though):

https://www.lexaloffle.com/bbs/cposts/un/undune2-3.p8.png

Also note that there is a "Code" button under the game window which shows the full source code.

Unfortunately, no, I don't think that's possible. There's a few null bytes used here and there, like marking the Lua offset, and your screenshot can't capture those, no matter how accurate the rendering of the actual bytes is.