1) If the XCF is still open in Gimp, then yes, use Edit > Undo (or Ctrl-Z). If you have already saved the XCF, or reloaded an exported image (JPG/PNG) then no (in some simple cases you can re-extract something from a uniform background, though).
2) Yes. But It should rarely be used, if ever. Flattening is done automatically and on the fly when you export to a "flat" format (JPG, PNG...). The "normal" workflow is to keep an XCF with all the layers/selections/paths, and export a JPG/PNG from that for "presentation" (web, print...).
2) Yes. But It should rarely be used, if ever. Flattening is done automatically and on the fly when you export to a "flat" format (JPG, PNG...). The "normal" workflow is to keep an XCF with all the layers/selections/paths, and export a JPG/PNG from that for "presentation" (web, print...).