Regarding cutscenes, I'd argue that "etiquette" would be going with the majority, given that you're free to watch them back at the Inn. If 5/8 people want to watch them, wait quietly, dance, run around spamming airquotes until they're done, whatever you like - but do it peacefully. If 5/8 people want to get on the dungeon, get on with it and watch the scenes later.

Regarding gear. Gear does make the player past CM/Prea/Ifrit. Sorry, but a GC/WP weapon has no place in a Titan HM, nor does class gear. By hitting commence you just wasted all the time the other 7 folks spent waiting to get in. "Maybe", and it's a big maybe, they'll be able to carry you through it - but you will not be contributing to that fight. It doesn't matter how good you are if the unavoidable damage is enough to kill you. You are, quite simply, undergeared - a liability, at best. The etiquette, in this situation, would be not to queue until you're actually capable of a fight.