They say "If you have livestock, you will have deadstock" . You have to accept that some injuries, infestations and illnesses will happen. When they do, it will generally be something very common which you should learn to treat. . Taking a chicken to a vet is not a good idea. Vets are best at...
I would put the chick in with her. It will have a better chance than if you raise it in a box in your kitchen and try to put it out there when it is a half feathered stranger and has no hen protecting it.
On the whole, the incubator is a lot more reliable than a hen -- especially an inexperienced hen. They very often get so excited abiut being a mommy when the first few eggs hatch, that they get up and leave the remaining eggs. I'm not saying they all do that, but the incubator never does that...
