I make it a point to keep broody hens separate during incubation as well as when chicks are off nest. Considerable strife can be realized when broody hens are commingled and likely factored into problems you observed.
Look into term imprinting and what is required for it to occur. You will find the terms "window", "exposure" and "duration". Then consider how hen with poor mothering skills relates to the imprinting process.
You are much more likely to have poor mothering when you have been messing around...
No. Hen knows chicks based on voice. Anything impacting chick voice can cause problems. Hen will ignore her chicks she can see but not hear but be very concerned about chicks she can hear but cannot see. A strange chick she can see but not hear is less likely to be attacked than one she can...