@jafd: update-gdkpixbuf-loaders is called during libgtk installation for sure ,you can check here. I have no idea why your system was left in inconsistent state (suspect that dpkg configure step has failed silently), but it is not that installation package is broken. And re vkb - CSSU does not touch vkb or hildon-input-method in any way. Did you try to reinstall it?

Re how the testing is performed - I have been using libgtk + hd + Qt4.7.4 from CSSU 16 for more than a month on my 2 devices (one of them primary phone) and also those were installed on one more device(again primary phone). Believe me or not but noone from CSSU team/contributors has any will to break users' devices or to put them in a reboot loop. Do you remember when PR13 came out how many users has broken systems? And that is with Nokia testing the update for several months.
Only I can say to you is sorry for you problems, it is not something intentional. And if you can find what the root cause of your problem is I ( and I am sure lots of other people) will appreciate that.