yes, gale was supposed to be the new karsus, you do hit the point.
In my walkthrough, Gale didn't want to take the crown himself for some reason. There were no convictions, no influences, just after the line about "Become a god?" he explained why he wasn't going to do it and... that was it. I never took Mystra's side in dialogs with Gale, never tried to convince him to give up anything, he just did it himself. I don't even know what influences that.
i think that the following video shows a very different dialogs with your encounter tav try hard to convince gale, and stop gale's ""ambition" finally.
