Hopped to the end so sorry if I am repeating. Medicate for this farrier visit, definitely, it is in the filly's best interests. Medicate by Vet for the teeth- my Vet, who is a qualified dentist and trained on Minis, will not touch any horses teeth without it being sedated, unless it is a quick rub round with the file and she knows the horse well- she is rarely wrong and would not sedate if it was not needed. When she did my geldings wolf teeth as well as a routine filing, I got a lecture (well, she is at a teaching hospital...) on how horses can feel the teeth being pulled/filed, whatever anyone tells you, and he would no more touch a horse for wolf teeth without pain relief than she would take wing and fly! I had never even thought about it, so it was food for though....
I agree completely that this filly is not necessarily abused- I am just now wading through my horses feet- two a day til my back gives out(!) My two year old, who has not been handled nearly as much, routinely, as she should have been , was as good as gold. One of the broodmares, who is handled daily at this time of year, is eight years old and was shown extensively, acted as if I was a Martian. This is a mare I bring in every single day as she is a founder candidate (never had it, but she is just such an "easy keeper" - read she is FAT!!- that I am careful of her) so she comes in and has her own feed that is basically grass chaff, and she gobbles it up- anyway, she is tied up and fed every day and here she is acting as if she has never had her feet picked up in her life. Other days she will run around screaming she has never been caught/haltered before and is a mustang- it is just her nature. The two yearling colts I have just brought into the stalls....well, they have quite obviously NEVER seen a human before, let lone been haltered!! (Every single day from birth to the present day they have been handled, that is all...) SO...what I am telling you in my long winded way is do not be to quick to judge, these wee ones have weird little ways, and my mare is very happy to be brushed and petted, but it does take a few minutes of "wheedling" to get a halter on her. The rearing? I take it as par for the course in a youngster, most of them act like hot air balloons most of the time, I work with it, it settles down in time, trust me!
Catch her, no matter how long it takes, tie her up, be calm and firm, do what you want to do and then turn her loose. No messing, no fuss, just as you would with a two year old child! You can put a breakaway on her if you really want but sooner or later you will have to take it off and start all over again, possibly, I will not leave any halter on any horse, I have lost two, I do not intend to lose another and breakaways don't always break away...