Hello. This sounds like a tough situation.
I would try your best to get a muzzle on her and then cut the harness off. You can use some string, twine, a leash or rolled gauze. Tie it first and then get it around her muzzle and pull it tight. Wrap it around the muzzle a couple times and that should allow some time to get some scissors in there and cut the harness free.
Valium is not going to do much when she's worked up like this. You can try giving 4 mg, but I think her stress is going to over-ride it and you're unlikely to get much sedation.
If all else fails, try to get somebody to pick up you and the dog to get to a vet. There, they can give injectable sedatives and then get the harness free.
Here are a couple videos on how to make a muzzle:
I hope this helps, let me know if you have any other questions.