1. "Writing probably first appeared as a way to keep track of resources, and maybe what was owed to others."
Agreed.
2. "because ideas and new ways of doing things don't usually pop out of nowhere"
Happens all the time. For a modern example, just look at various technologies.
3. "Little confused as to why you do not consider customs and beliefs as religion?"
I have focussed on writing but the real emphasis belongs on law. No law, which binds physically, then no religion, which binds in other ways. Of course, you can still have deeply held beliefs and persistent rituals, but they do not bind like religion does.
4. "There is ample evidence of belief in a spirit realm through the art they have left behind, and of course burial goods in grave sites makes it pretty clear they also believed in an afterlife."
Agreed, but see #3.
5. "Having a conscience is part of morality."
You can program a computer to make moral decisions, yet those choices do not show conscience.
Young children can have consciences, yet they can't have morality. Thus, the law holds them to a different standards over crimes that would merit grim punishment for an adult.
6. "we might not feel comfortable with our ancestors morality, but that doesn't't mean they had no concept of it."
You presume the existence of a sophisticated, organized form of thinking called morality. Morality bears at least as much resemblance to law as it does to conscience. In light of this resemblance and in the absence of law, morality can not possible emerge.
I wish to emphasize that conscience has attributes that can appear to us as morality, but the two are not the same. A society with people of conscience can exist -- even thrive -- without morality.