I have all collar fix mods, plus achilles rebalance and USM which one is messing with my robes' defenses?

My guess would be the one that tweaks the games settings. The USM didn't really have robes in it. The collar fix's were just graphical.

More than likely the rebalance mod... but what exactly did it do? Did it up them, or lower them?

My balance mod doesn't affect items. You said "all collar fix mods" which is a little vague. If one of the these mods includes changes to robe properties as well as appearances, then...

Troubleshooting 101 tells me that if you find a_robe_02 in your override, then you simply need to find which of the mods you downloaded also contains a_robe_02 to determine which mod is giving you problems.