Interesting questions. But are you sure that these traits gave a survival advantage at the time they were selected away?
For example, humans don't have the ability to generate Vitamin C, even though some of our ancestors were able to do so. But if their diet was rich in foods containing Vitamin C, then a mutation that stopped production (or changed it) would not be detrimental.
Would a dominant carnivore who ate little or no cellulose become less likely to reproduce with a malfunctioning cellulose metabolism? probably not.
Also keep in mind that traits evolve in parallel. A neutral change like a slightly less protruding brow could have evolved along with a beneficial change like a bigger brain. Females may connect the smaller brow with the bigger brains and prefer such partners, which would give the smaller brow a selection advantage.
I will admit that evolutionists are better at making up stories than most people. This is probably because this is mostly what they do for a living and "practice makes perfect" as the saying goes.