I wouldn't say I'm an expert GM, but I've done it a lot and have a lot of "theory" down. That said, I have played under a lot of new GMs, and I think the best thing is wait until the end to give any advice unless they look absolutely stuck and that the game might crash and burn. It can suck and even be a bit boring if they don't really know what they're doing, but you can also help by going along with what they planned (i.e. don't go the complete opposite direction). I guess basically what I'm saying is I've been frustrated or bored or tired, but I like to think I'm pushing through it so that the GM will get more experience and eventually be able to GM more so I don't have to (as originally I believe I was the only person in my current group that had GMed before).
I'm not really sure where I was going/what I said, but hopefully it helps...
I guess to briefly answer your questions:
No,
Maybe (if it interrupts the game/bothers the GM),
See above