Good luck with that! It was pretty much broken in Squeeze, and was removed afterwards. At least I couldn't get it running reasonably on vanilla Debian back then.
Some people have tried to get Hildon running on newer frameworks (I think Aapo and Android_808), but I don't remember if that yielded any satisfactory results.

As a first step you could try to get at least Matchbox running. It worked in principle in Wheezy, but there were some major flaws in the UI design. I believe I found it impossible to return to the desktop by means that are available on a N900.

The 1st major point why I haven't replaced Maemo on my Main N900 yet.
The 2nd would be battery life. I got about 6 hours idle with a battery that runs Maemo idle for a week. I'd want at least a day (with suspend2RAM if incoming calls can wake it up).

No idea about the rest, but forget Gnome and Unity! These are way too heavy. LXDE works mostly fine, so should any window manager. I guess Enligtenment and Mate might be ok too. But I doubt, even Xfce would make sense.

Not at all! Otherwise someone will open another thread just like this one in two years.
Any changes that are necessary would have to be integrated into Debian, to maintain them in the long run.

No, because I don't think it's feasible.
DebiaN900 is a great playground and wicket has my utmost respect for putting it together (just like qole, who pretty much made me buy a N900 just by creating Easy Debian), but in the end the "open" N900 is just locked down too much, to fully revive it with a purely community-driven OS.