AFAIK I know demonetized videos (which are 'unsuitable' for YouTube ads) generate zero revenue, so I think that's a no. It would go to creators who create exclusive series for YouTube premium, though.

I remember seeing something about a very low percentage of viewers being YouTube premium, but accounting for something like 40+% of revenue.

That depends on how much ads pay, how many users watch ads/use adblock, how much they watch etc. Videos that get few ads or low-priced ads might indeed have the membership be a large portion of the revenue. Generally it's more in the 10-20% range though.

Yes. In my YouTube studio analytics I can see the percentage of revenue that comes from YouTube premium subscribers.

Yes. I believe the split is the same as with ad revenue - 55% to the content creator and 45% to YouTube.

Almost. They say "a majority" but don't specify the exact amount. So, over 50%.

The standard split YouTube does with their creators is 55% to the creator and 45% to YouTube. I believe they would use the same split for YouTube membership revenue.

Yea, but for Premium it is different. At least, back when it was called YouTube red, they just promised a majority.