But I can't recall if your opponent can apply Counterstrike or even Defensive damage back onto Grayseer Thranquol or not? Is he immune due to attacking from a different Zone instead of the Battlefield?

I'm drawing a total blank on this...d'oh!

Of course not, he isn't immune.

Yeah, I was nearly certain but for some reason last night - I was suddenly wondering. Duh (on my part).

He is allowed to be declared an attacker from another zone other than the Battlefield, but he still participates in combat so is a legitimate target to any damage effect that involves combat, but does not specify the battlefield zone.