Well it's true enough that without access to the multiverse and a tardis, we can't really know the pre-election likelihood of this outcome (a concerted push for gun control).
I did (and do) believe that new gun control was going to be a priority of a reelected administration given the dynamics I thought I saw in late 2012, and I expected some tragedy or other to be exploited as political leverage. That seems a more or less standard strategy used to advance gun control proposals.
If you believe the particulars of Sandy Hook are driving the push for gun control, then hindsight bias seems a reasonable conclusion. I agree with your point that changes in the details of this specific event might make it more or less "exploitable" so to speak. I disagree with you insofar as I believe the likelihood of some "exploitable" event occurring to approach 1 over a president's term.
I think the long term dynamics driving the political interest in gun control are independent of the Sandy Hook incident itself.