one of the possible reason is that Apple committed a large volume of iphone 5 screen so they need to use up all existing screen order, before they can start to mass production new version of iphone with larger screen. it is not technical issue but purely logistic management issue they need to resolve.
I don't think so, there are so many ways to consume the small panel, e.g. iphone cheap and ipod. Also, if two new models are released, why don't they try make a large screen for iPhone 5s? The selling price of iphone 5c should be HK$1000 less than current one. If iphone 5s has larger screen, I don't mind buying it with higher price, e.g. HK$1000