Perhaps others will have more answers for you but...
I don't think the helipad is accessible but I don't know. Which deck is it on? In any case I never stumbled across it. They did have to use it on the Inaugural because someone had a heart attack and had to be airlifted out. But that, of course, doesn't answer your question.
During the Inaugural sailaway, my sister and I went to the Promenade trying to walk the whole thing. But you can't - near the front of the ship it becomes sort of walled in (but with windows cut out) - they were storing extra carpet there and it was a mess. At first we thought maybe that was just another Inaugural problem and kept walking, but we came upon what seemed to be a crew area with crew members welding or something so we turned around again. So unless this was changed later and the deck made available, the answer is no on that one.
Yes, it is a helipad...I never saw anyone (other than crew) in the bow area. The Promenade Deck allows walking from stern to about the front of the superstructure but again it is "crew only" toward the bow. I walked back and forth on the sunny side of the ship on days "at sea." There is a running/jogging track up around the funnel--I think it might have been about 8 times around the track to the mile.
You will enjoy the "Millennium" as it is well-designed; food is ok; shows are ok; and service is quite good. Enjoy your cruise!