Only leave the halter on in the field if it is leather, has a leather headpiece or breaking piece, or has some other form of breaking method in the event that she gets caught on something. It is not worth the risk of her getting caught, panicking, and not stopping until something breaks (and with a solid nylon halter, the first thing to break will likely be your horse)
Here is a field safe halter with velco breaking point Amazon
.com: Horseware Field Safe Halter: Sports &..."> Amazon.com: Horseware Field Safe Halter: Sports &...
Here is a breakaway halter with leather crown Amazon.com: Reinsman 1" Break-Away Nylon Halter...
Here is a breakaway halter with leather piece at the snap Kensington Breakaway Halter with Lead from SmartPak Equine
About pulling back and other tying issues...my 13 year old does the same thing, he starts eating the post and pulling back. I can't do much about him biting everything in sight because he's just curious and HAS to be doing something...you could probably invest in McNasty or another no-chew spray on product, but she will likely find something else to do.
As far as pulling back, the only success I've had is to tie my horse in a rope halter with a long lead, like the kind all the NH trainers are using, and wrap the lead around the pole a lot of times to create a whole lot of resistance but also give slowly when he pulls back. The bigger bite of the rope halter discourages him from pulling, the wrapping allows him to have a tiny bit of give so he won't just keep panicking, and having the rope be so long allows me to wrap it a lot of times and he generally remains tied when he's done pulling. Definitely wouldn't leave a horse like that and walk away for a few hours, though, because you will want to be there to take the slack back up when they're done pulling back and also loosen the knot on the halter because it gets tight. (helps to get it a little bit wet, I keep a spray bottle with water in my grooming box)