Couple of thoughts. It won't hurt to put some more eggs/brood into the nuc. You can probably spare it and the future population will help get the nuc up and running. Keeping the nurse bees on the frame and even shaking a few more in wouldn't hurt either. I wouldn't worry about the penned up robbers. Yes, they might go back but foraging bees from any other colonies will quickly find your nuc anyway. Personally, I would stop feeding and make sure the entrance to the nuc is small enough to defend. Keep an eye on them (I kind of hope for a day or two of rain when this happens) and keep an eye on the queen in the nuc for sure. I think you can salvage this colony.