From the time mine were little I threw a handful of feed into the brooder and kept a little on my finders every time I went to pick them up. They pretty quickly calmed down and dot used to being handled. At this point, they are pretty tame.
Another note about processing that I'm not sure about- if chickens are like mammals it's important to keep them calm before butchering. For example, I know that if you shoot a deer that has been running or stressed (or ran a distance after being shot,) the lactic acid built up in the muscles...
My 6.5 week old Barred Rock Roo has less meat on him than a chicken McNugget. I've never raised JG or Wellsummers but I can't imagine they would be worth much till at least 15 weeks.
I gave my chicks a pan of dirt to play in, they were able to scratch stuff out of it and they seemed to get all the grit they needed. After aabout two weeks I started putting them out in the tractor for limited periods on warm days.
I am still trying to figure out this chick grit thing. Is there a reason not to put a pan of dirt into the brooder instead of buying chick grit. Mine are in the brooder during the night and go outside into a tractor for the warmest part of the afternoon to range. They have had a pan of dirt...
I wouldn't add anything until you see a problem. Some people use a vitamin suppliment in the birds water (Aviacharge is the name, I think) but I don't know how early they start.
Animals bred to fight (Game Birds, Fighting Dogs,etc.) sometimes seem to "turn on" on become aggressive suddenly. I don't know as there is always an obvious reason.
Does anyone stun them by swinging them hard against a tree? This is how we do grouse when we hunt them and don't kill them with the first shot. I would think this would make the whole process easier. (particularly if you are going to have to do it without assistance)
We had a RIR like that when I was a kid. She never seemed to have troubles eating and drinking and lived for several years. No idea if there is a correction for this or not.