She's getting enough calcium then. It's good she's eating all of that. Keep it up!
It sounds like she is trying to wean the kittens. Moms start nursing less when weaning. Are the kittens eating wet food and kitten chow well? If so they should be okay, though many moms nurse for weeks and weeks, even when the kittens are eating solids, though not as often as before.
No. She needs to gain weight first. It's actually good that she's starting to wean though, because she needs all her reserves.
I wish she weren't so picky. Maybe feed her small portions as often as possible of what she does eat. You can pick up Nutrical paste from a pet store or your vet. It's high calorie, and will supplement her as well. The beef is good, because it has extra iron. Have you tried eggs yet?
It may take her quite awhile to fatten up, but she will eventually have a heat cycle, so be sure not to let her out. Poor gal. I'm sorry I said spay. I momentarily forgot how much she needs fattening up.