Help!! I have many questions about my breeding cockatiels

Ok, i have a female and male cockatiel bird. The male is about 3 1/2 years old, and female is 1 year old.
-why does my female cockatiel go on top of the male cockatiel and mates? isnt it supposed to be the other way around..?
-She lays many eggs, but they never hatch, even though i see the male in the nest box a bunch. When i know the egg is dead, i remove it, crack it open to see if it was fertile, and most of them have had an embryo, and the last one i took out had feathers. (I take the eggs out when they start to smell and i know their dead)
-i feed the cockatiels the seed mix from the pet store, and tried soo much to get them to eat fruits and veg. but they refuse, i want them to be healthy so i have been giving them vitamin drops in their food and water. what else can i do for their heath?
*please help me with my questions, i really care about my cockatiels, please help..thanks!