1st red. You're missing the fact that there is twice as much chance of a GB combination (as BG also comes into this category)
2nd red. Surely Vice-versa?
3rd red. No. As two - 2 child families have, by definition, 4 children, the chances are that there are going to be 2 of each gender. If family A has (at least) 1 girl, the chances of the other child is a girl is one in 3. ie of the 2 boys and 2 girls, 1 girl is accounted for, leaving 1 girl and 2 boys.