Had a look at NG's video and given how much she was labouring my initial thought was that it must be something not infectious or rather transmittable (bacterial, viral, fungal or whatever) because for an adult to be in that kind of distress, I would have expected that the chicks wouldn't have lasted that long had they been similarly infected. I didn't see anything that looked like frounce, but of course there wasn't much chance to see anything. It did look like water blood on her breast feathers, which could have meant she was in a fight and what we watched in the video was a wet bird with blood on her breast trying to expell a pellet (that open-close mouth behaviour is reminiscent of the process of expelling a pellet. If she'd been injured on the breast, it could have been painful or less than efficient and hence it looked more laboured. Or I thought it might have been a reaction to something she ate, that her body was trying to fight it and that is "blood" on her breast might not have been blood but rather vomit so to speak. Or perhaps something she ate was sharp enough to pierce her esophagus or crop or something similar preventing her from being able to expell anything. That would definitely be a problem for a raptor.
I don't know, and I haven't heard anything not already posted here, so that was just my impression when I watched the video. I do hope they find NG - alive so perhaps she can be helped, dead if she can't, because this behaviour would be something I would want to know the answer to, in case it is something being used in the environment or just an accident or if it is an infection/parasite/fungus of some sort.
Just my two cents.