Can Garen use W while cc'd? If not maybe that is a buff he could need for his late game.

Why would garen need a buff? I guess he's not really played in higher elos much, but he has around a 50% winrate last I checked.

That's one of the few mastery point of Garen. If it was useable during CC that's what would be done 99% of the time. There wouldn't be that guessing and prediction element to it.

Why does Garen need a buff?

If he could do that it would make him even more tedious to deal with, if you catch him or CC him he'd not only shrug it off but also take less damage.

If Garen were to ever need buffs, it wouldn't be to his damage or survivability.

No he can't, and that would certainly be a great buff, and not only for late game. It would be much useful to reliably shield during a trade in lane phase, even if the opponent has CC.