Did I "really" have too many points for the following bid?
I'd rather bid 1d there, not 2d. I think it's as likely you have game as they do; unlikely in either case, really.  But you're discounting your partner having, first seat, something like ten points and a five card spade suit.  That's 4S in the bag, if the points are useful ones (not wasted QH or whatnot).  6-7 spade tricks, a few in diamonds and clubs, and you're all set.  You don't want to miss that! 

Really, you have more like a 14 count here anyway - with the diamond length and the void, your hand is worth more than a normal 12 counter.  Bid 1d and see what happens - you might end up in a makeable 3NT, 4S, or who knows what else.

If your 4th seat opponent has a 12-14 count, they're probably not in game anyway - as you said, they have 7 opposite them on average, right? - and so you're not preempting anything useful.  On the other hand, if they have an 18 count or whatnot, where a game is a likelihood, your preempt was pretty useless in my opinion - they can double, get some information from their partner since they have to bid over the double, and be in just as good of a position as if you hadn't bid at all.

One other thing: if you're going to do this regularly, you'll need to annotate your convention card to indicate that your weak 2 range is 6-12 3rd seat.