Interesting question: whose side does Heinrich take in this scenario? (Of course, whether it matters depends on the date, because Heinrich was so much younger.

True. BTW, should we tell cahn about the younger sibs, or at least about the Heinrich Situation? Anyway, as you say if Fritz returns as long as Heinrich is still a kid, the point is academic, but if FW holds on to life long enough, I'm thinking Heinrich goes with Augustus Wilhelm, who was his favourite brother in Rl anyway, plus given the age gap between him (Heinrich) and Fritz, he wouldn't have had many memories of him, let alone those inclined to make him take Friedrich's side.

Wiki reminds me that Heinrich had contact with Hamilton and was under discussion for ruling the new US of A; now that would have been an AU...