Well, one key difference would be that in 1757, AW was still alive, so the "brother" referred to must be him, not Heinrich, even if he was in disgrace and casheered, because there would have been no getting around the fact that he was the next in line for the throne and unlike his son an adult. Now there's a thought I hadn't had before: Fritz gets captured at Leuthen instead of achieving his most glorious victory. AW is just about to go to Berlin from Leipzig where he hung out with a recovering Heinrich. Unlike Heinrich, AW would have been capable of saying at this point "to hell with these instructions, I think the war's lost anyway. MT, you can have Silesia, and also, we surrender if that means the war is over instantly. I don't see what Fritz can do to me that he hasn't done already if you send him back." (Otoh, presumably Heinrich would talk him out of the surrender.) But really, the Fritz capture plot has to happen after the summer of 1758, i.e. with AW dead.

To get back to the abdication subject, figures that Fritz pointedly does not include this option in his instructions.(And for all his talk to Catt and others of how after the war, he'd have abdicated and handed government over to AW if AW hadn't died/intends to abdicate after the war and hand things over to young FW, there's just no way he'd have done it.)