Yes, the Eyecon Driver G makes a "Grateful Omega Drive" sound for a basic finisher. But, as this video quite clearly illustrates, when all of the Parka Ghosts are summoned, the finisher IS called the "Mega Omega Formation".
I hope this clears things up for you. All of the Parka Ghosts = Mega Omega Formation.
Yep. Just like how in Who Loves Heart the Most? they had the Drive Driver announce a finisher during a Tire Blending, while omitting the "Full Throttle" announcement. The sound guys are only human, and prone to making those types of mistakes.
Just because something didn't happen on the first use does not mean nothing was left out - as explained above, for all we know not all of the Parka Ghosts were summoned. It could also be that the Mega Omega Formation and Omega Drive are 2 different finishers. Grateful Damashii has only been in the show for 2 episodes (+1 of Zyuohger) as of this writing (as Ghost 24 and Zyuohger 7 have now aired), there is a lot of episodes left that could show the other finisher.