I know quite a bit about Bane so I’ll gladly answer.
1.His father was assumed to be dead/missing which is why he was not present in jail and why little Bane was sentenced for him.
2.They knew his age and because of that when he was young he was put in a secluded area with Osito (His Teddy Bear). When he got older they removed him from there and put him with adult prisoners. He was only a teen though so he was still quite young but they didn’t really care.
3.IKR. They were basically asking for disaster but they were also looking for someone strong enough to survive the venom.