Is the ablution valid when one wipes the head, then moves the hand away to remove excess hair above the ear, then returns it to complete the wiping? And what is the ruling on a barrier that was not known about until months after performing ablution and ghusl, and what is the ruling on past prayers in this case?

The action mentioned does not affect the validity of ablution as long as you have wiped your entire head. Some scholars are of the opinion that it is permissible for a woman to confine herself to wiping part of her head. As for anything that prevents water from reaching the skin, it invalidates ablution and prayer. Missed prayers must be made up, and if their number is unknown, one should estimate what would absolve them of their responsibility.