If the mother hen is attentive to the chicks and is clucking and teaching them to eat, then I would leave them with her. I would keep them away from any other older chickens till the babies can fend for themselves. She will keep them warm and teach them what they need to know.
I would because you can control their safety.
Mama can take care of them just fine. You might want to screen off a little area for them, where the other chickens cannot get to them & hurt the babies.
No,they should away be with her,she's the one who will keep them warm or unless you have you have an incubator.
No you shouldnt unless the temp is under 96 degrees if it was me I would just encase