Tuxedos with more than one button should button all but the bottom button.
Best $90 I've ever spent.
Eggshell? Depends on the color of the accompanying woven loafer; call for details.
Those ties must be about 15 feet long.
which is why he wears 6-buttoned coats, buttoned all the way up, to cover the nub that only goes halfway down his sternum.

Merril Hoge is my fashion icon.

He'll be wearing his Army mess dress, so his jacket will look somewhat like that. The groomsmen will all be in tuxes though.
there were several more links using..."tuxedo etiquette buttons" as my search...