Well I can certainly see the concept of a useful competent character getting gimped out and pointlessly killed off as funny. Hell, I've seen some instances where it was downright hilarious.
This was funny more because it was such an idiotic move by DC. DC actually thought that killing of a popular, though minor, character would get more attention and bring in more readers than it would annoy. I know people who just about gave up on DC about 5 years back, in part because of "Identity Crisis". The story itself was just plain crass. (In other words, I am laughing at DC, not with them about the story.) "Identity Crisis" indelibly smeared Sue Dibny.
In the case of IDW Galvatron, I want to see the character destroyed on many levels, just so no writer will want to use them again for years, if at all. Your scenario for this needs a bit of tweaking, but is a step in the right direction.
I suppose it could be one of those "it was all a dream..." type stories. That makes sense. It's just unclear to me what the writer was going for.
I read that story as Mirage having a fever dream, and possibly being a bit nutty. He is definitely having doubts. Given his place in the Autobot ranks, he could definitely do a fair amount of damage to the team. He might even be able to help deliver a fatal blow depending on exactly when he revealed himself.
Note his reaction when the last of the Autobots are being killed. He is not entirely happy about it. It looked like a question of Mirage wanting the war over, but wanting to avoid needless bloodshed. He likely would want the war over as it would be an expensive distraction from other things he would want to see happen.
Prowl's theory does not exclude the story tying in with AHM.
Mainly because we know so little about Mirage in IDW continuity that I have nothing to compare with the evil version in the story. We don't know what his beliefs are, and whether or not they're dodgy.
I tend to think that the best way to look at IDW Mirage is "like the old one, or close enough". Start with the old characer profiles, and work from there. Mirage has doubts about the Autobots and the war. He is smart enough to reflect on these, and question his role. As James McDonough said years ago, Mirage and Thundercracker have a good deal in common.
I don't dislike it per se, but it doesn't do anything for me. It's more like "this is what an evil Mirage would be like" and doesn't tell us anything about the genuine article, except that presumably he's nothing like the evil guy in the story.
The idea is that both have doubts. Mirage would not be much happier if he switched sides and became a Decepticon, as shown in the story.
Dom
-wants a G1 Mirage figure.