Seed mixes should be their staple diet as its designed with all the right things they need. The rest should be given sparingly and as treats. Your birds may refuse to eat certain things but it will only be for a while. Their natural instinct will over take them eventually. If the only food available is seed mix, they will eat. They wont let themselves starve if that is all is on offer. As for the time spent out of the cage? As much as possible, keep them in their cage too much and you run the risk of them becoming aggressive and fearful of human contact. Besides if your cage is only a standard size they need to exercise in a large space. So the more time spent out of the cage the better.
Just a little story I thought I would add. I had three cockatiels once. One was friendly the other two (purchased together) were untamed. I never did get them tame, they would peck at you if you went anywhere near them and become very aggressive. But a starnge thing happened one day. I had bought them a toy to hang in their cage, attached was a piece of string in a loop, i thought nothing of it. The next morning I found one of the birds had got its head caught in the loop and could not get out it was almost strangling itself. I tried to get hold of the bird to help it, but it attacked me and became more strangled. I got a pair of scissors to cut the string through the cage. It was only Just in reach and had to try and cut the string from an awkward angle and distance. It was just not quite working the bird for ages kept moving along its perch away from me. But it was on tiptoes and would surely have strangled itself to death.