First, does she already know how to sidepasses, TotH, and TotR? If she does, you'll have to teach her these on the ground first. I can go into greater detail if she doesn't know.
Backing I'm pretty sure she knows. Start on the ground. You should be able to stand in front of her, walk forwards, and say back. She should respond by backing at straight, without any pressure from the lead rope. Carry a dressage whip and tap her between the legs until she starts to back if she at first doesn't respond. Never touch her with your hands or use halter pressure unless necessary. Once she is backing nicely on the ground (this both helps undersaddle training and judges will sometimes ask for backs on the ground), you can move undersaddle.
Ask very, very lightly, and only ask for one step at the time at first. First, use only the verbal cue "back" and recline your body. If she doesn't respond, pull lightly on the reins. She may not respond immediately, but soon will get annoyed with the mouth pressure and back. Release your reins to slack. Gradually ask for more and more steps. With each step, release all rein pressure. Soon, when you say back, your horse will back with a verbal cue and continue backing until you say woah, no rein pressure necessary to keep her moving backwards. Judges LOVE that. It keeps your horse happy (no head flipping or gaping) and looks NICE.
This may sound very basic, but going back to the basics with something like this works miracles. Trust me.
After you have a good, straight back, back though everything. Stop randomly on the trail and back. Back up hills. Back through creeks, over tarps... If you can do it frontwards, you should do it backwards too. Judges love to ask for you to back over strange stuff.
To keep her straight, use heel pressure significantly behind the girth if she goes astray to push her hindquarters back into the proper position. Once you get your TotF, you should be able to back patterns, like Ls. You should be able to change directions while backing with no problems. Soon, you'll be backing beautiful squares and triangles.
I actually ride in an English saddle, so I can't really help with the endurance saddle situation... The Western saddl will probably have to go though. How heavy is it?