I'm just another one of those. You know, those who do things. Things that need to be done by those that do these things in order to do those things that don't do these things themselves. Has the word 'things' lost meaning to you already?
Will this C'thun start at 6/6 again? If it does, will it receive the +2/2 from this card?
It starts at what ever your c'thun buffs got to. So Yes +previous Buffs and Yes.
I don't know why people haven't understood that C'thun's buff are a Player Aura that always attach to your C'thun no matter how it become yours. So if you have +10/+10 on your C'thun it get +10/+10 Wherever it is, whether its killed, revived, stolen, copied, copied as a 1/1 (base stats change but not the buff) played from hand, summoned from deck, Silenced, Stuffed up someones arse then put on the board. Doesn't mater whether its a first one, second one, eighth one, doesn't even matter if you never had one in your deck to start with and an effect gives you one somehow.
Doesn't mater when the buffs are played either, whether it's was before he's played, after he's played, before he's even in your deck, Wherever he is doesn't matter.