Basically this. The worst of the damage is right in the beginning of the fight; I promise, it tapers off as the drakes die.
Personally I HoS one add tank, have the other Beaconed, and directly heal the Halfus tank, but that's all personal preference. Regardless of who you use it on, make sure to utilize it. It can save wipes (and has, for me).
Move from Fireball Barrage. If you want to help with raid damage you can focus the Proto-Behemoth and pop Aura Mastery when he starts to do Scorching Breath.
Most of all don't panic. I know you want to make a good first impression, but there's a reason they invited you in the first place. They wouldn't have done so if they were unsure of your ability.
Your spec is identical to mine, so yay for that! lol
I would honestly switch out Glyph of Cleansing for Glyph of LoD though. You can sneeze and hit 6 people with it in a 25-man.
You WILL go through mana like it's water in this fight. Just don't panic, and manage your mana as best you can.