Some desktop environments don't work anymore after system changes (reusing `/home` in new install)

After some reinstallation/configuration/partition resizing work (I really can't say where the point of failure was, but see below for details), I ended up with unity not working anymore, a situation essentially similar to this or this: login works, and then I just get a blank screen.

I've tried several things listed in those questions, like restarting various stuff, purging and reinstalling compiz, unity, and ubuntu-desktop, or deleteting their config files. Also using compizconfig-settings-manager and re-enabling the Unity-plugin didn't help (although it was disabled, in case that's relevant information).

I then just tried to install alternative desktop environments and display mangers, and that's where things started to get weird: namely, it turned out that lxde and openbox do work under both lightdm and gdm, while unity and gnome-shell don't work on either (after login, just a background image appears and nothing more).

One additional thing that sometimes happened in lxde was that an "Internal Error" box showed up, containing (among others) the following information:

Title: xorg crashed with SIGABRT in __libc_message()

CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins

(but this only showed up after I already reinstalled and changed many things, so it might be related to problem or not).

Now, I have no idea where the failing is actually happening, but I suspect compiz...

What I actually was doing before this happended:

• Installing 14.04 on a separate partition (1) from the original installation (2) (also 14.04)
• Normal login, so the default unity, lightdm and ubuntu-desktop did work
• Installing some programs
• Mounting my old /home from the old installation, which is on a third partition (3), as /home for the new installation (and setting this in the new fstab)
• Installing more stuff, and deleting some config directories/files from ~ which I thought are not needed anymore
• At some point here, I have also used unity-tweak-tool, but only to increase the number of virtual workspaces
• Resizing partitions (1) and (2)
• Reboot

After the last reboot, said things happended. I don't think the partitioning stuff plays any role, but I mention it for completeness; but maybe my reusing the old /home did something weird? On the other hand, I (think I) rebooted at least once with it mounted as /home of the new install, without problems.

I also use a dual-monitor setup, but that has never caused any problems.

The question is: is there anything else I could do for getting unity back and gnome-shell work? Or should I just reinstall everything -- in which case, should I not just reuse the old /home, or would that not be a problem?