Just wondering if the CS is open source bc there are a lot of things I wish I could fix/tweak and if not then why isn't it?

No. It's Bethesda's property mainly, but there are also a lot of features that are a result of licenses from other companies, I believe.

Why would it be open source? Why would any of Bethesda's tools be open source? They are a commercial company.

Yeah, the third party software in the engine would certainly complicate releasing the CS source - in fact if I remember correctly it prevented Bethesda from releasing a converter that Morrowind had although it's been so long I may be mixing things up.

Even if third party software wasn't an issue I'm not sure if Bethesda would benefit terribly from making the CS open source given that they haven't released any source code for the game itself. Sure some things in the editor could be tweaked or fixed but I'm not sure if that would translate to the game itself very much.

Any fixing you might want to accomplish can be done through a OBSE plugin. It will, however, require a significant amount of reversing.