Since I was trained English first and recently started Western in the last several years, I will say that from personal experience, Western is a whole 'nuther thing from English. The seat position IS different, lots, and legs and butt and feet are used differently than when I rode hunter.
Here's my thought on the trainer and her suggestions. Do it. Do it until you understand what she's trying to accomplish and don't agree with her. Right now you have no clue, and neither do I, so I can't comment on what your trainer is telling you. I wouldn't have lasted 10 mins with that one who was all impatient and wanting to yell and fuss, I promise you that. But to switch trainers and then come on here and second guess everything she's told you is a waste of time and money. Either get all in or don't do it at all. Then, once you understand what she's doing and where she's going, if you have something you don't agree with, address it with her. I, personally, don't care for a Kimberwicke and I think there are lots better bits. I also don't know everything there is to know and I know there are a lot of people who use the Kimberwicke. So, if you have an issue with a Kimberwicke on your gelding, tell her and tell her why you do and ask her why she wants you to switch. And if you agree you need to switch after she explains it, then ask her if there's something you can use besides a Kimberwicke that will accomplish what she wants to accomplish. Same thing with the training fork. If you don't like it, tell her and tell her why and ask what she hopes to gain from using it. I don't mind training forks for accomplishing certain things, maybe your horse has developed a bad habit and a training fork will help correct it quickly.
The main thing is, either get with her program wholeheartedly, or don't waste her time and your money.