Do you think in the past Brandon may have been more like Willoughby when he was younger??

It's possible, but I think more likely that he was always more steady and responsible. As to why Brandon would be attracted to Marianne (sensiblity) instead of Elinor (sense)seems more like the attraction of opposites. I am fairly introverted, and my former husband (& several former boy friends) was/were very extroverted. I always felt I was seeking someone to balance me, and that may what JA did with having Brandon attracted to Marianne: her intense emotions would balance out his more banked fires.