on point 2 - to the best of my knowledge the code hasn't been added to any VCS, it definitely hasn't been added to the official SCP repo.
As for point 4, the split occurred before I was a member here but I think headdie has pretty much nailed the issue that lead to the fork, i.e. the SCP was slow to commit WCS features, or was rejecting WCS coded features (with good reasons or unjustifiably?). Additionally after the release (which I was around for) there was some negative feedback provided (valid or otherwise?) here about WCS:DD which further soured the already poor relations. The reasonableness of all this is a point of debate, but it's one I don't want to get into as IMO it's past the point where anything constructive could come of it
If you want to port the WCS code over to FSO, I'd say go for it and don't worry about the history. From my point of view, it'd be great to have Linux/OSX officially supported, and maybe even multi-player restored. Those alone would be worth it
(although lightshafts/shadows/etc would also be supercool