What part does meritocracy play in a caste and class system?

A very small one. In most cases caste and class are based on who your parents are

It provides a small lubricant to allow the machine to function better. Those who study history believe that this was the major difference between England and France during their long wars. It was not until the French Revolution that a Frenchman could rise by merit.

Nothing it's garbage. In capitalism as well.