Yes, you can unlock before it's activated. You need up update the firmware first. Use Apptap to update the firmware - or do a hardware reset and plug it into iTunes. And then use ibrickr to activate (move the lockdown file over by hand, or use an automated method) and then unlock.
I'm not sure if visual voicemail will work with an AT&T SIM. You could search here:
http://www.hackint0sh.org/forum/forumdisplay.php?f=123 and see what the gurus say.
And yes, you can unlock it and use your old SIM.
As far as the restore - well, no, it won't undo the unlock. Which is good for most of us, but not for you if you want to return things back to exactly as they were before the unlock. There is no un-unlock that I know of - and I doubt anyone has written one because none of the hackers would want to do this.