Hello again!
First off, I think the best advice I can give is don't panic! You're doing fine! And I'm happy to help and answer questions any time.
If she is female, and gravid, how do I treat her? I've heard that they give birth to live young...
Yes, you are correct, the bitaeniatus are live bearing chameleons and if she is gravid, there isn't much you can do. Offer her lots of healthy, gutloaded bugs. Give her proper temps and humidity. Try to keep handling to a minimum. All stuff you would normally do with a chameleon!
... so would this mean she would deffinately be fertile? (She has been housed with males and females.)
You betcha! Maybe not though. Again, I have to put this disclaimer on; I'm only commenting on your animal from 4 pics. I could tell you for sure if I saw her in person. Ohhh, my kingdom for a teleporter! LOL!

I digress, a lot of literature about the bitaeniatus says they breed multiple times a year or don't really have seasons in which they do mate. I haven't found this in my collection - for me they are seasonal breeders. I'm a firm believer in cycling my animals and I'm sure my bitans have fallen into the rhythm along with all my other bitan related critters.
How can you tell the difference from male and female?
As I said before it can be tricky with the true bitans but there are subtle differences with their tail base. Looking at the last pic you posted, I think you've got a female. I'll dig up some sex comparison pics of my bitans for you as a reference... as soon as I find my CD! Argh. I'll try and find my notes too, but gestation for my ladies was about 4 months.
When he/she is in her cage she goes pretty much black and white with black kind of squares.
Slightly stressy colours, but normal. I wouldn't worry about that, beyond letting her be.
Remember, chameleons have been giving birth for thousands of years, they know what to do. As much as it's cool to watch them give birth, I let my girls be on birthing day.
Hmmm... I've probably created more harm than good with my advice and thoughts eh.
Good luck! Don't be shy about questions either.
Cheers!
T