The mirror allows a lot of light to the autofocus points that are behind it, so not all of it is reflected.
The reflex mirror allows a small portion of the light through for AF. It makes no difference to your eye, since your eye has the greatest computer on earth letting you see the image.
The sensor reflects quite a bit of light (I don't know if enough), a nano coating could be optimized to increase the reflectivity at 45 degrees.
The sensor reflects some light, but that is a natural result of having a highly polished surface. (It's like when your smartphone is turned off, you can use as a mirror in a pinch, but you wouldn't want to replace your bathroom mirror with it.) It is not the same as being a mirror. Any coating the increases reflectance will decrease the amount of light passing through it, therefore making the sensor less sensitive to light.
Live view would work just like now, the sensor would go into vertical position.
So how is it AFing again? I thought you said it used dual pixel?
The sensor would not be attached at the edge, but it would be through the middle, it would pivot on this axis. It would move half the distance that the mirror does now.
That might help a little... But one there is another issue. Your mirror box will need to be longer, unless you expect Canon to drop the EF lenses altogether, and make a whole new line based on the new, shorter, registration distance.
Vibration seems to be the biggest problem to me.
That is an understatement, I believe. There is a lot more in the sensor than you can see through the lens mount...