Try checking your antivirus settings so they aren't blocking the Gallery.
But you may need to do a full 50-50 to find the content that's breaking the Gallery, given some of what you described happening:
Organizing the Mods files into bite-size batches (with any files they require, like XML Injector or a Core Library mod or mesh) and test one batch at a time. For each test, delete the file called localthumbcache, then put only that batch in Mods.
If your game behaves as you expect, set that batch aside as "cleared" and delete localthumbcache again.
If it doesn't behave as you expect, take out half the batch and see if you're closer to finding the culprit. Remember to delete localthumbcache each time.
Keep in mind that having duplicates of mods, having old and new versions, and having two mods that do the same thing can also cause problems. If, when you start putting multiple cleared batches back, you start getting problems again, there's some kind of duplicate or conflict issue happening.