Are you looking for org-babel?
Maybe, because I personally enjoy Emacs, but Babel cannot gather code from external files into a single place and then keep them in sync. org-babel can "tangle" source code blocks into an outside source files, but this makes the org document the authoritative source and pushes it outwards. I want something that treats the actual source code as the authoritative source. (I haven't actually used "tangle" though, so I'm open to correction.)
And no, because Emacs isn't going to be popular enough to be widely used.
Yes it can. org-babel-detangle copies edits you make in a tangled file back into the matching source blocks in the Org file. There's also `#+INCLUDE: "file.py" src python :lines "10-40"` and org-transclusion package - `#+transclude: [[file:foo.el::some-defun]] :src elisp` shows live content from an external file in the Org buffer.
Babel has no built-in way to import arbitrary existing files, but for the stuff that you make it make sense, it knows how to get its shit together back into one place.