How do C'Thun's buffs work if I Entomb it?
Case 1 and case 2 are to all intents and purposes, identical.
The C'Thun buffs act as an aura on your hero. So let's say to start that the druid played C'Thun as an 8/8. After you entomb C'Thun and he plays another buffing minion, his effect will now say that C'Thuns he plays are 10/10. This is broadly irrelevant (since he doesn't have any more C'Thuns to play), with two exceptions:
- Minions that have conditions based on C'Thun's attack will inspect the aura effect. So his Klaxxi Amber-Weavers will now have 10 health after the latest buff (even though the druid no longer actually has a C'Thun in his deck).
- If the druid obtains another C'Thun card, through Discover or other card-generating mechanics, then he'll be able to play it as a 10/10 (or whatever his current buff level is).
Meanwhile, if you draw that entombed C'Thun, he'll just be a vanilla 6/6 since you haven't played any effects that buff your C'Thun. In case 2, when you draw the entombed C'Thun card, it'll be whatever your personal buff level is. (Just as when you draw the C'Thun card that was in your own deck to start with.)
In short: the buffs to "your C'Thun" could be better worded as buffs to "C'Thun cards that you play", and are persistent effects on the hero that plays them, rather than buffing specific cards.
C'thun buffs are treated as buffs to the player, in a similar fashion to Dalaran Aspirant's Inspire-based Spell Damage. Once you Entomb the enemy's C'thun, they have no C'thun in their deck (barring shenanigans with Unstable Portal, Thistle Tea, or the like), and their minions will no longer provide any effective buffs. (If they do have another C'thun, they will continue to buff it.)
Unless you have previously buffed your own C'thun, the Entombed one will also not benefit from their aura and will revert to 6/6.