Infinite Unstable Evolutions

It turns out to be pretty simple, as long as you know the trick. Card played have an ID in the zone log and if that ID is above 67, the card was created by some other card or effect. This won't work if the card is played, returned to the hand, then played again, but there might be an easy way to track this as well. If nothing else, you could still track the position of the card in the hand.

You cannot have more than 10 mana at once, so the only way to play a card more expensive than 10 mana is to reduce its cost to 10 or below. So, no you are not missing anything. The way to work against hunter traps generally is to deduce which trap it most likely is and play around it. In case of freezing trap it's using an expendable minion (or one with a good battlecry) to trigger it or use a card which destroys secrets (e.g. eater of secrets)

However, I don't see how this is overpowered. The likelihood of this situation occuring is fairly low and if it happens, you know it is a possibility and have to play around it. There is also the possiblity, that the hunter played misdirection and your own C'thun might have hit you in the face. Taking those two traps into account, it's an easy decision not to attack with C'thun until you have another minion to trigger the trap available or a card to destroy the secret, as the risk is simply too high. So unless, you are in a desperate situation anyway, there is little reason to take that risk.

Additionally take into account what other trap he could have played. Playing an explosive trap, when you only have a C'thun on board is utterly pointless, unless you'd die from the damage; the same applies to dart trap. Bear Trap only makes sense, when the hunter survives the hit from C'thun. Snake trap pointless as well, unless you want to trade, which is unlikely when using C'thun. Snipe? What's the point with a C'thun on board? Apart from that freezing trap is by far the most common hunter secret. So, you can see, unless the hunter was utterly desperate, there was little reason for him to play anything besides freezing trap or misdirection.