Hi,
Poor ewe it must be painful but silly sheep at the same time.
I guess you did right in starting the antibiotics if there's discharge, however a couple of questions: did you clean/inspect the leg before padding it in the pipe? Might you have missed a splinter of bone protruding from the skin? That would certainly get infected, could explain the discharge. I don't understand how there could be discharge anywhere if the site of fracture didn't have broken skin
I have treated sheep with splints on their legs and although they find it uncomfortable to move, they quickly adapt and start wobbling on it. Have you checked that the pastern cannot move? If there's space for the slightest movement in the cast between the piece of pipe/padding and the leg, it won't heal as quickly and more chance of splinter of bone protruding. Also make sure the duct tape is not cutting into the skin, and the top edge of the pipe too. Double check the padding hasn't flattened as that will make the whole assembly not as stable.
I don't know what your experience in bandaging would be but if you try bandaging your own leg (even with just rags), you'll soon find out which tension is firm enough to hold on the splint without cutting the blood supply.
It would also help if your vet could give you some painkillers for the ewe - but I guess that sometimes is not an option in case you decide to have the injured animal slaughtered instead.