Scow2 said:
The crucible of ME3 was NOT a Deus Ex Machina. It was the Plot-macguffin.
Just to be hideously over-technical, I don't think it's the McGuffin because
what it does is actually essential to the plot, and a McGuffin is an object whose only function is to provide motivation.
For example, let's say I write a story about some people trying to break into a vault and steal a diamond. Is it important that what is in the vault is a diamond? No. It could be a priceless artwork, or a pile of money, or an experimental microchip and the plot would remain largely intact because the story isn't about the diamond. It's about how the characters break into the vault. The diamond is just there to make them want to do it.
Now, if the characters breaking into the vault was just part of the story, and in fact they needed the diamond to build the lens of a laser to fight aliens or something, then the diamond is no longer a McGuffin because it does more than provide motivation. It always needed to be a diamond, or at least something you could build a laser out of.
The goal of Mass Effect 3's narrative was not to find the catalyst, but to stop the Reapers. The catalyst was established very quickly as essential to stopping the reapers. While I totally agree that it wasn't a Deus Ex Machina, I also don't think it was the McGuffin. It was just a plain old boring plot device, as far as I'm concerned.
I also think, in addition to what you said, it was pretty heavily foreshadowed by implication from the very first game, simply from the fact that the enemies are a massive fleet of kilometer-long giant robot space squid who require an entire fleet of conventional ships to bring down just one of them. As I say every time this comes up, it was always going to be a plot device. It was always going to be a virus, or a superweapon, or Shepard overwhelming the Reapers with the power of his/her love for Garrus. It was never going to be "Shepard shoots his/her gun and the reapers all die", anyone who thought it was wasn't paying attention.