Scoutrider: It is quite intimidating haha! Especially when I know she has little to no respect for me. I recognize that, I just don't know how to change it. I also had not thought of using a rope halter, it won't leave rubs on her face? I've never used one before, but I will try one out this week for sure! Also, the last time I stuck my toe under Candy's nose she leaned on me and pushed me flat on my bum! Maybe I'll try again and be a bit quicker on the move this time haha! The lunging is without a saddle and without me... this is why I don't think she is in pain. We have not had her seen by a chiropractor, just the vet. I don't have problems staying on when she bucks, I am quite used to it by now, I just don't know how to make her stop it! Lastly, everyone at the barn I ride at rides western and many of them cannot relate to what I'm saying with their quiet calm quarter horses haha!
Your girl sounds a lot like my friend's QH. He's learned that he's the big cheese in his horse/human herd, and he's had it proven to him that he can pitch his weight around and get his way, especially with certain people. Of course, he's only a (rather chubby) 15.2 hands!
I've never had a rope halter rub one of mine, but I don't leave them on - only for groundwork, or in the early spring when their full of it and not inclined to listen as well as they should when leading from point A to point B. We have miserable winters, and they unfortunately get shorted on turnout during 6 months of blizzard.
I personally like a softer, floppier rope halter over a stiff one, but a stiffer rope tends to have a little more "bite".
For interrupting her grazing, you need to be very quick about it - get in, make your point, and get out. I would also hold my hands up, as close to her raised-head eye level as possible, to deter her from entering your space. If you're going to do any groundwork, I highly recommend starting in a grass-free arena - 1 less distraction for both of you.
She may not be in saddle-pain - thats why I recommend a chiro's evaluation. If her back's out, then she could definitely hurt even if she's "naked". By the sound of it, though, even if she is in pain that isn't her only issue.
Vets can tell you a lot, but sometimes it can take even more of a specialist to pinpoint a problem. If your local library has it, take a look at Linda Tellington-Jones' Equine Awareness Method - there's a section on fairly simple equine massage and probing for pain and stiffness, maybe you could do a little yourself and find out if she's over-sensitive in her topline, etc.
Again, by the sound of it, you're rather nervous about working with her on the ground. You might benefit from finding an on-site trainer to work with both of you and help get the ball rolling.