I think the internal plate width actually changed slightly with the 11s, but not a lot.

One one bike, I have a hybrid cassette, half 11s (1.6mm sprockets), and half 9s (1.78mm sprockets). I have narrowed spacers to compensate for 11s spacing.

The cassette runs fine on both the 9s and 11s sprockets, with only extremely mild issues at the 9s/11s transition.

I also have been using various cranksets with 11s, including 9s and 10s, without significant problems. Skating?

I can't say how your shifting will be. It should shift, but one issue is that as the derailleur moves the chain leftward, it would hit the edge of the sprockets before it hits the shift aids on the next sprocket over, so results would be unpredictable.

As far as comparing chains. Wippermann has posted a summary of chain tests that they've conducted now with 10s and 11s. And, the Wippermann 11s chains apparently last longer than the 10s chains. However, they don't indicate an internal control being used between the two test series, so one can't necessarily equate the two.

I've chewed through several Shimano 11s chains. I think they do actually last longer than the HG73 9s chains, but I've been replacing them quickly as I have a great fear of tearing up the rest of my drivetrain.