Replacing light switches-- can all live / all neutral wires be grouped? Is conduit a valid ground?
To answer your questions in turn:


Your future state with all the always-hots nutted together and all the neutrals nutted together looks spot-on, yes.
You are correct -- metal conduit is a valid equipment grounding conductor (always has been and pretty much always will be save for some extreme circumstances that don't show up in residential work), so grounding to a metal box works in your case.  Make sure to use a 10-32 machine screw into the matching hole on the box (vs. one of the mounting holes, which won't be tapped properly for this) -- you can get a bag of pre-made-up grounding-screw-pigtails, even.
They're behaving like a normal switch here, so they'll figure this out on their own I reckon.
That other orange wire is simply an always-hot running to some other switch, receptacle, or outlet on the circuit, likely in some other room.