This link should answer your first question:
http://www.chekinstitute.com/articles.cfm?select=26
basically all it's saying is this:
"To protect the shoulder joint capsule from being stretched out or injured the exerciser must determine how far to safely lower the bar. It is essential that each person determine optimal bench press range of motion for his or her own shoulders, as each person is different."
I find that some days I have more range of motion than other days. Even after stretching. On those days I just lower the bar to where it's comfortable.