There's an episode of Mythbusters where they manage to get two guys on the door from Titanic, so they concluded it could be done. Thing is, it was a tricky balancing act. When one tried to climb on, usually the other would get knocked off. It took them so long to figure out how to get them both up there, by that time they would have both died of hypothermia. So I don't care what they said, I considered it busted from what I saw.
James Cameron commented that it doesn't matter if two people could fit on the door in reality or not. From his storytelling standpoint, only one person could fit on the door. That's the way he wrote it. So if that failed to come across on screen, that was an error of execution. The story says only one could fit.
As for Jack, he was a poor artist who drew a lot of naked prostitutes who gave him sex without having to pay. Then he gets on Titanic and pulls an attractive rich girl. So from that standpoint, he sounds like the broke wandering "cad" type that some might call alpha.
What damns him in places like this is he falls in love with the girl (horrors!), and ends up playing the white knight and sacrifices his life for hers. So many here will call him a beta. In that time and place though, he did what was expected of him. Would more guys here admire him if he would have kicked Kate Winset's fat @ss off of there and crawled up there himself?