Yeah it is definitely a concern exposing your flock to something they may have picked up, if you do end up taking them back definitely quarantine them for awhile away from the others, at least a month maybe longer depending on how bad the injury is. It sounds like they do a pretty good amount to...
Hmm complicated situation. I think if they are in a not completely predator proof area, I personally would take them back and give them a safer place. They may have other duck friends there, but since you said they were with eachorher before they even went to live there, they are probably more...