Majority wish or want does not drive morality....... morality implies good of the majority whether the majority wants it or not..... in fact, much of majority wishes and desires seem to turn out to be not good for them eventually.
No. It took Moses to redefine morality for humanity as they ventured off deeper into ignorance and immorality. Ignorance and morality are natural in everything and everyone. I think it's based on survival. It's a dynamic that needs to be rebalanced periodically by God's Avatar like Christ or Buddha and Moses.
Yes. Your definition does not mention "majority" as a condition to establish morality. And traditionally, it's been a minority that has dictated morality to the majority of most cultures, be it political leaders, religious leaders, aristocracy, or whoever the majority has de facto allowed to lead them. What the heck does atheism have to do with it?
Yes it can, morality is relative, absolute morality is just a fantasy some theist got, they usually need to cherry pick text to fit their supposedly not relative moral to keep that fantasy going on 2018.
There are two sides to Mortality, innate and learned.
Learned is what you describe.. The majority think slavery is ok so it becomes morally justified. Unfortunately this is easy to hijack and distort for self-interest so change comes very slowly. Slavery is the big example, but sexual orientation and narcotic consumption are continually changing in what the public defines as 'moral'.
Innate is your empathy you feel for your fellow human being without any learned prejudice. When someone trips and falls across the street from you, ever wonder why you feel bad for them? this is innate morality it is an evolutionary product of a social animal.