1 - do the two notches under the coverplate against the shaft signify anything like up/down or are they just molding bits?
They're just there to keep the assembly from rotating once it's installed...they don't serve any "mechanical" purpose. Bear in mind that you'll have to notch out the top of your wood or plexi to match these nothces.
2 - does it matter which way up they go?
Nope. I have mine side-to-side, but it doesn't really matter
3 - which of the gold and silver contacts is the earth?
The silver are the "common ground" contacts.