How to keep caramel from hardening prematurely?
I'm sorry, but what you are asking for is not possible. Caramel is a hard substance (I mean pure caramel here, not the stuff which has added dairy or acid and never hardens). You cannot even keep it melted the way you can do with water, because its decomposition temperature is lower than its melting temperature - at temperatures at which it is liquid, it is not staying chemically the same, but progressing to charcoal. 

So, you have to buy more molds, or make your recipe in several batches. But you can't just keep liquid caramel around until your molds are ready to reuse. This would be breaking the laws of physics.