If i entomb the enemy c'thun then play a c'thun buff card will I buff both of "my" c'thuns?

Yes. Imo it's a dumb mechanic.
I think if you entomb (their) Cthun.. then they play buffs to buff (their) Cthun (No matter where it is).. it should buff the one in your deck.

But it works like you're saying.. it becomes (yours) apparently and only (your) buffs work for it.
Works well if you're running a CThun deck also.. but if not, then it's pretty lame.

Does it retain any buffs the enemy gave it?

Not from what I recall. He played his at a 12+ and when I drew it it was a 6/6.

I think you will like this

Good video, thanks.

I thought the Doctor would thank me for that :s

Thanks!