Player rating for tournaments across all installments of Heroes.

I'm not missing this point; I just think that, based on your idea in point 6, the coefficient should be higher the more high-ranked players participate in the tournament. However, in practice, things might turn out differently when applying such a calculation.

And that's exactly what I'm saying. If it were reduced by 1/3, I would think twice about whether to participate in the Lumumba tournament, even if I met the minimum rating requirement.
There, you can fail very reliably :) But for a weak player, it's already beneficial. The aging effect from their meager rating is negligible there, and the bonus from the tournament's strength will definitely offset the loss.