If it was a belt or a belt tensioner problem, you'd hear it. They have a way of letting you know when they're not able to do their job properly.
Anytime there is one or more pullies that are not keeping pace with the rest, the belt will scream at you.
It could be several things, but I'd have the alternator checked out first. Okay, scratch that. I'd have the battery load tested first, then have the alternator inspected.
Other things to check are any and all battery/alternator/ground connections and wire/cable quality. There is no reason at all for your voltage to drop that low, even if it was because of the pulleys, unless you were parked with all your accessories on for an extended period of time.
My guess is that your battery probably has a cell going bad, and the new pulleys reduced the alternator's ability to keep it hidden from you. Who knows? Tests will be necessary, and a battery load test should always be the first.
As far as the p/s pump, it could be heading south, too. Changing the crank pulley may have slowed down the p/s pump's rotation just enough at idle to expose the fact that it's beginning to fail. I agree that these may seem like an odd coincidence, but at 13 years of age, things start to fail in a car. Installing the pulleys may have saved you from the aggravation of becoming stranded on two different occasions later on this summer.
Or, it could be Gremlins from the American Cyanamid plant in Bound Brook, but I hear they have them under control.
Take care,
-Chris