Could the leg be broken? Maybe not a complete break but a fracture of some sort? The swelling looks considerable. I don't say this to cause you more concern as you have enough obviously but just for you to consider if you hadn't already. Guess it's an unknown without an xray? I agree you should get a second opinion asap.
I do wish you & your horse well and I can relate to your anquish about her condition. My horse is suffering badly now with a month old severe laceration to his left hind and an iffy prognosis.