#1. I have read that kittens should be able to eat as much as they want, since they are growing. Once she is close to adult size, you should give her between 2-4% of her total body weight.
I've read the same. Kittens need a lot of calories since they grow so fast, so there's really no reason to worry too much about them putting on weight until they're done growing. If it were me I'd just give the little one as much as she's willing to eat
I can't give you any advice on supplements, because I don't use any. My two cats are getting about 60/40 raw food and canned at the moment, and only because I've got one stubborn older cat. I'm going with prey model feeding right now, and the way I make sure they're getting enough of everything is with lots of variety. I have over half a dozen different kinds and cuts of meat in my freezer right now, and that's not including organs. I don't grind the meat either. I feel like it's much easier and possibly healthier(?) to let them eat a variety and get all their nutrition from the food itself, rather than grinding it up and having to add supplements back in. It can actually help dental health and reduce plaque build up if the cat gets the chance to chew a bit. They can even chew bones, like chicken wing tips and flatties (the part of the wing that contains two small bones) or quail are about the right size.
I'm currently just using meat straight out of the super market, but once I have my chest freezer I'm going to bulk order hormone/antibiotic free meat from a local specialty butcher. I haven't had any problems with super market meat, this is just an ethical decision for me. I think just about any meat fit for human consumption isn't going to be contaminated, so just buy from where ever you would feel comfortable getting meat for yourself. And make sure it hasn't been 'enhanced' in any way with spices, broths, or anything else (although this is just common sense).
You should see if your little one would be willing to help you out with all the grinding and just chew some of her food herself
My older cat needs her meat chopped up fairly small right now, but the younger cat can handle a whole turkey thigh, half a quail (bones included!), chunks of chicken breast and just about anything else I can throw at her. For now you could just put a few bigger chunks of chicken thigh in her usual ground meal to see how she takes it. Or try adding something new - beef stew meat is a favorite with both my cats, although I understand some don't tolerate beef well. I love it because it comes pre-cut!
From what I understand younger cats are usual pretty adaptable, so it's a great idea to get her used to chewing while she's still young. My older girl has taken a lot of work to convince that she needs to actually USE her teeth
But I've noticed a difference in her kibble-caused plaque with just the little bit she's chewing right now. Even if you still decide to grind most of your cat's meals a little chewing on her part can save you from needing expensive teeth cleaning for her down the road. Good luck, and I hope some of the extremely knowledgeable people here can help you out with your grinder questions!