Are you compelled to do the MQ with each char?
That is only about 5% of the total content.
Of course not. In my current game, in fact, I'm in over 150 hours of play and I haven't even touched the main quest. That's hardly the point. Once I decide I want dragon shouts, I'm forever saddled with random dragon appearances. Once I decide I want to enter the civil war, it's the same thing (you need to help the Jarl of Whiterun, which activates the dragons). Even Oblivion allowed for Obvlivion gates to stop appearing, and that was forced. I'm asking for something optional.
And it was your wording that confused me. "If it is triggered after the MQ is completed, then you have already done that with the previous character." does not make any sense to me on any level.
