I sense a great deal of confusion in you. Right because there are plenty of posts that explain how the payouts work, I'm surprised to hear you say that.
First of all:

Also, why are you making a conversion of the author SP payout using the median price? The system doesn't do that. In all 3 cases the SP portion would be 0.75.

Wrong. See what happens today to my post:

Kind of a funny coincidence, the system used the steem median price to calculate how much SP my post has made.

The value of SBD is fixed at $ 1,00. What changes is the vote value.
The higher the values of steem, bigger the amount of the vote.
The higher the values of the vote, bigger the amount of SBD earned (every time pegged to 1$).
The higher the values of steem, lower the amount of steem earned (higher steem median price).

And while it's true that the lower is the value of steem, lower our post will earn, it is also true that the lower is the value of steem, lower authors will contribute with their posts and so the reward pool will be divided among less posts which will earn more.
The amounts of steem created would be always the same but divided among less contributors (that in the end will earn more steem).
This is my last answer and I hope that we have finally sorted this issue out.