Well bute now is probably not going to do anything. Whatever damage has been done by inflammation was already done within the acute stages of injury, 3 weeks ago.
Where on the leg is the swelling? Does it move with the skin or is it attached to the structures in the leg? If you pick up her foot and try to move the swelling around, does it go with the skin? Is any part of the back of her leg sensitive to palpation while the leg is picked up? Does she flex sound? Does she test positively with a hoof tester?
I can't imagine a vet having "no clue". I think it's time for a new vet, preferably one that has a clue - these are all basic diagnostics that are very easy to perform and can easily pinpoint the source and cause of any swelling.
Further diagnostics are usually not needed, but treatment can range from stall rest to basically icing it twice a day and riding lightly, all the way to surgery if it is a broken splint bone.
In short, new vet and a proper exam should set you on your way.
They say money doesn't buy happiness -- well happiness doesn't buy horses!