I'll focus on question #2 -
You said "I've tried with jhbuild just yesterday and only some modules were built, but gnome-shell and gtk3 weren't included in those that built" and then asked
"Can jhbuild gnome-shell install script be fixed in order to allow a clean build?"
I would think so. First, http://live.gnome.org/JhbuildDependencies/Ubuntu tells you what Ubuntu packages you need installed - use the Lucid installation, keeping in mind the comments from the Maverick section.
Then - what is the moduleset in you're .jhbuildrc? The default is gnome-3.0 so I'll assume is that. The rc file points to the moduleset you'll be using.
Usually people doing a full "jhbuild build" skip certain packages and put that skipping in their .jhbuildrc. You can read elsewhere, but building mozilla within an environment just to test gnome is something most people skip.
Actually, if you're having problems, I would suggest you not do an argument-less "jhbuild build" at all. "jhbuild build" without arguments builds everything in your moduleset, which I'll assume is gnome-3.0 (look in your .jhbuildrc to see). Better to do "jhbuild build gnome-shell", which builds only gnome-shell and its dependencies (I believe gnome-shell currently has a total of 36 dependencies in jhbuild - that includes a count of not only dependencies, but the dependencies of the dependencies and so forth). Once it's built, you can do a "jhbuild run gnome-shell" to run gnome-shell using the libraries etc. you have built.
I should note, if you're doing a jhbuild off of recent code, it can be rough sailing, although it's still the easiest way to go if there's no PPA for Ubuntu that you like. I just did a fresh jhbuild. gtk+ broke on some calls, so I browsed git, saw that that had been changed in the day before I installed git, so I git reset that directory to one from 24 hours before. I then worked. Also, the package I wanted to build would not build. I searched the Bugzilla page for patches which updated some calls which had been deprecated long ago and recently removed by the gtk+ dependency. I applied them and it then my gtk+ 3 dependent built. So it was not automagic, I had to fix two broken builds. But it finally built.