Well jeez, so thats the "real" reason there is that green neutral bus bonding screw, so you can use it as a neutral in a main, or as a hot when you remove the bonding screw.. haha
In regard to the first topic, you're point about a 2pole breaker being able to be on the same phase if you used a 3ph panel on a single ph service, well some single phase panels can have a 2pole breaker installed and it will be on the same leg for each pole...
i'm not saying using a 3ph panel on a single phase is right in any way though.
I've seen a picture if it done though, doubled up wires on phase "B" for a jumper to phase "C", in fact i think there was a pic of it on the violations forum here many moons ago.
