Only a GM dealer can do updates for the Volt (which actually don't occur with any frequency). But it's PIA, as you have to take your car to a dealer, and we all know what that entails. But then again, I've had my Volt for more than a year and no updates have been issued.
Hard to know. But it's also pretty clear that the Model S firmware controls many more functions than the Volt's, so it's got to be much more complicated. Some people have had issues with the Volt relating to charging, car shutdowns, and the like. But judging by posts on the Volt forum it's not been a major problem and doesn't occur with much frequency. I've had no issues.
*All* GM cars get a mandatory firmware upgrade when they go into service for anything but the basic oil or tire treatments. My wife's uncle tells me they get software updates pretty regularly for every model.
I wouldn't be surprised if they extended the current OnStar telemetry capabilities to allow for an occasional push of software. My guess is that the cellular data limitations across the entire fleet (and the fact a LOT of GM owners don't subscribe to OnStar) are what keeps them from doing this regularly.