Orochi is a shogun and was never implied to be a sword-god or anything like that, it is completely arbitrary to expect a normal political figure like him to be a major powerhouse. (Yonko aren't political figures in any traditional sense at all.) Selecting random unknown figures to hype up into main opponents (although they were notably left out from any such suggestion) is indeed one's own mistake. There is always a possibility, but one should be able to move on from it, if it turned out to be incorrect.
It is not arbitrary to wonder about Kaido's son. I don't think he can be like Momo or Orochi because... there are those. And Kaido wants to see him. At the worst he can still be like Momo, for deliberate parallelism.
The remarks about Oden's pact in boiling are just dumb as heck. Kaido didn't crush the scabbards, so that fulfils any sensible obligation one should expect as a reader and in reality, they are not given forever-peace and the kindgom of their choice (to ensure that peace)... And no, Oden didn't get to keep his sword to take a shot at Kaido if he wants to at his execution... So arbitrary.