I don't think that anyone can go on the observation deck. You have to have eaten at CG that night, in order to be able to use the deck.
And yes, it does get crowded. It was PACKED last week; we were lucky we had a table by the window, or else we would have had to go reserve a standing spot 20-30 minutes before the fireworks started. If you're eating at this time, without a window seat, it can be a real problem.
As someone else pointed out, while they do pipe the music in, they don't dim the lights. Many people just kept talking loudly, and the reflections of the interior lights on the glass did interfere a bit with the fireworks view. We had, for the first time EVER, though, a LOVELY window table with a gorgeous view of the castle & fireworks. Even the loud talkers & reflecting lights didn't bother us a whit.
Cheers!
Heather W