Alex, this is a question which I believe you are the right person to answer. If one day all mammals started habitually laying eggs (like chickens do), where would we draw the line at what eggs are acceptable to eat? Would we eat any of these mammal eggs? What would the moral and/or ethical difference be between eating a rat egg and a lion egg (if any)? This haunts me at night and I desperately need answers.

This seems pretty straightforward to me. People would eat the eggs of animals they already eat, like with chickens. Generally speaking if someone says they enjoy lion eggs just back away slowly.