Endiku, thank you very much for that info on Quest dewormer.
I have accidentally overdosed my 350 pound 10.2 pony on ivermectin, pretty much exactly as you described - the dosing ring slipped.
I did recently deworm with Quest, but was very careful and pony got the correct dose for his weight.
But I had no idea that the tolerance for Quest was so much lower; I'll be very vigilant in the future.
To the OP, I agree with what the other posters have said - sedate for the farrier. Also agree that she'll need a series of trims, a couple of weeks apart, rather than doing it all at once. I took in a pony once whose feet were almost that bad and he was sound on them, amazingly. But when we started to trim him back he was REALLY ouchy and sore.
I might also give a little bute each day until you complete the trim down process.