Are Developmental Mistakes Essential to Evolution?

This is pure gibberish. Mutations don't have any effect other than their consequence in expressed form - a mutation that alters protein folding is just a mutation like any other. And biologists generally hate the concept of "evolvability", which usually (as in this case) has no mechanistic basis.

The word 'mistake' implies intention. There's no such thing as a mistake in evolution, any more than there's such a thing as a 'failed' scientific experiment.