If you look at my topic title, you'll have an idea of part of what's wrong with my game. Little "s"s before words, skill & attribute desigations changed, enchantment values skewed, some enchantments not working anymore, can't enchant anything new anymore. Happened after I had been in the CS making a new NPC & tweaking some previously made ones. Ran everything through TEStool, it reports everything clean. What did I do wrong? How to fix?

Sounds like you may have a bunch of GMST entries in your file which shouldn't be there.

Open your esp in the editor and check the details.

Retraced my steps, realised that I had accidentally clicked the "combine loaded plugins" option in the CS. That's where the problem started, but I've no idea how to undo it.

I don't think you can... you'll have to delete the combined plugin and re-download the individual plugins methinks.

As Theta says, I think that bug stems from unwanted GMST entries.