3. Yup. I put a little note in there when I changed them... Jan 2012.
Duracell AAs. So they're less than a year old, and the LED light was strong and steady. And we don't use it all that much.
The radio receiver was getting power via the USB port; I reconnected the receiver to the mouse several times to make sure it was connected. I pulled the plug on the receiver and plugged it back in several times as well. I also pulled the batteries out, waited, and put them back in to make sure there wasn't some kind of error loop in the mouse.
I manually changed drivers; I found ones for MWIE (no "2"; just MWIE) that dated to 2006. The WMM5k ones dated to 2012.
I didn't delete the WMM5k ones; just told the computer to use MWIE ones instead.