My expression randomly changed to the default and I want to change it back. However, there is no section to change the mouth in the beta wardrobe and the edit avatar page won't load.

I'm having this exact same issue. :/ Sorry that's not much help to you, just figured I should chime in as it's definitely not just you. (I've seen people mention this problem off-site on places like timblr too)

edit; aaand now my default mouth is gone and so are my eyes? okay then

As I said on the thread just below this one, I've seen people suggest this link, you could try that?

I used that link but I could only change the eyes after and the avatar edit page still won't load.

The avatar edit page has been disabled, I think. There might be a thread in Problems & Bugs about it but I don't know that there's another suggestion available, the link I gave is the only solution I've seen :c