Replying to Interchangeable FKS parts

I have a pair of pivot 15s from back in the day. I already replaced one of the broken heelpieces with a fks 155, but now the other heel piece isnt registering a din so now i need to scope out a replacement for that one.

my question. If i have the increasingly hard to find 15 din model, can I get away with using a 18 din or 14 din heel piece if thats all i can find for replacements?

second question: my other fks 180 pros have the plastic half moon piece. will i have to worry about breaking this piece? I have a tendency to break the shit out of bindings, so should I look into getting metal half moon pieces for these bindings?

thanks