I have never had children. What would you say the most important thing about being a parent is?
I think the most important thing is to remember that your words and actions will stay with them forever and to be kind.
Would non-blood relatives have a significant bearing on what experiences the child remembers throughout life be included?
Yes they would and that is why parents need to be mindful of who they expose their kids to.
How can parents reconcile differences in parenting styles if there is a disagreement about what types of people their children are exposed to?
Hopefully parents have that figured out before they have kids or it can be cause for some issues. I would only hope that two parents could agree that their kids shouldn't be around people that treat them bad or teach them bad things.
At what age should a child be considered grown up enough to be independent of their parents intervention?
As soon as they are legally considered an adult and parents are not legally responsible. 
What if the child is of lower I.Q. and does not have the mental capacity to make lifestyle choices on their own? Should the parents still be responsible after the legal age of adulthood?
Yes that would be a different circumstance.