Because adding CG to everything would cheapen something that is clearly not MEANT to come across as perfectly polished, using a lot of visual gags.
Exactly. Without things like the zombie getting the script out (I loved that bit!), this series just wouldn't be the same.
Since there is clear Valve fanboyism going on here, it probably is the G-Man. Sure, he isn't usually displayed with static, but chances are the static was just to make it hard to recognise him. Then again, since that would be too fucking obvious ("It's too fucking obvious"), it probably isn't.