I wish I could ask the stewards, why wasn't Button given a drive through for taking Alonso out? Clearly he simply understeered into Alonso and even though there was so much room, he did cause a collision which was "avoidable".
With a lot of the race still to go, it was deemed to be 'investigated after the race.' Why is that?
An in general answer to your questions:-
Protocol as defined in the Sporting Regs deems that all "incidents" are investigated after the race
unless the race director is certain there was a breach.
On the incident between Button and Alonso:-
Yeah, it was Button's fault, but, I agree with the officials on this one - and you know I'm Tifoso through-and-through

For me, racing as these guys do (and as I want them to do) implies they need to push to the limit, and, that inherently means sometimes you will push a little too far or hard - that's what I call a racing incident. You just got to accept these things will happen, you don't like it when it happens to you, but it happens. As long as its not something really deliberate or stupid. So, for me, yes, Button's mistake, but, that's racing, no penalties should be applied.
On recent similar incidents:-
BTW, I would have put most of the Monaco incidents in that category as well. Lewis shouldn't have been penalized the way he was - yes, the incident with Massa was clearly Lewis's fault, but, again, that's racing - it should have been left at that.