Nice code.

All right, I'm completely missing how this is coded. What's the key?

6805681
The key is that Twilight doesn't mispell or misuse words. If something sounds obtuse or looks wrong, it's intentional. If you want the actual key, someone else has to post it. She uses the same code in her later letters, but this one has the hint to solve them. I am not going to post the code, but I don't mind if someone else does. Its a very simple code. I was going to have this continue with more and more complex codes, just for fun. If I do keep writing it, that will be my motivation.

6805681

It's a really easy code, actually. Don't look at the spoiler if you want to try and figure it out for yourself.

I quickly figured it out because of how awkward and forced her choice of words is. It's an initialism. Take the central three paragraphs and jot down the letter at the beginning of each sentence into Notepad, and there's the message. Not much to go on, though.

7235299
Cool. I didn't know what it was called. It's just something I threw in for fun. Thanks for posting!