Well, you posted a nice little video showing some cog movement, but your logic is flawed. Moreover, you did not closely read what I said. And, yes, before putting a brand spankin new cassette on my WH-9000 C24's (recently repacked hubs, in fact), I sprayed lubricant on the rivets on the two carriers to no avail.
With that non-working solution in mind, follow my method to isolate:
1. Experienced creaking in ALL gears, when putting decent pressure on the pedals.
2. Noticed lateral movement of cassette. This applies to the entire set of 11 cogs, incidentally.
3. Removed locknut and inserted two extra washers.
4. Tightened locknut to proper torque setting.
5. Observed lateral movement eliminated entirely.
6. No more creaking in any gears.
I might go on to surmise that the tiny bit of play could lead to the 2nd carrier failures posted here. Oddly enough, I do not recall anybody posting pics of the first carrier rivets failing. Your results may vary.
I disagree that the creaking comes from the rivets. The cassette will creak on pretty much all cogs. I think I found a solution, though. Evidently, on many freehubs the CS-9000 will not fully cinch down in stock form. If you pinch the largest cog and move it horizontally, you will notice a tiny bit of play. This leads to the creaking!
My solution is simple: add a shim or two to the locknut, which compresses the cogs fully and eliminates the play. I used old Shimano washers from other 11sp locknuts. Works like a charm. Figured this out today after installing a brand new DA cassette on my C24 9000 wheel.
It absolutely is the rivets.
Your observations prove this; the 'horizontal' movement you are feeling is not due to the freebody being too long for the cassette, rather it is exactly the play in the rivets on the largest two cogs I was talking about. It is not due to the freebody being too long. I have never seen a freebody that is too long to allow for the cassette to be tightened down hard, especially from Shimano!
I have more info on this which I found the other day. Will be making a video on this when I get the chance, but to summarise;
Taking the cassette off, and holding the top two cogs section, you can feel play in the rivets by twisting the cogs against eachother with your hands. On cassettes that have a lot of kms on them, you can actually see the moving with your eyes.