Should a Freezing Trap return a stolen minion to the hand

Only those limited to one turn.

Mind Control and Mind Control Tech both take control of a Minion permanently. This is regarded by the game's rules engine as that Minion newly joining your army and thus follows the same attacking restrictions as a Minion you summon or play from you hand. That means, unless it has Charge, it can't attack that turn.

Shadow Madness takes control of a Minion for one turn only. This would simply not do much if the Minion didn't receive Charge implicitly. Actually, there has been quite a discussion about the decision not to write it on the card (supposedly to keep it simple), but it seems Shadow Madness is intended (and probably handled internally) to give Charge to the Minion.

From a logical point of view, this exception makes sense because the card would do very little without that mechanic (notably, it would work the same way it does now with Charge Minions, and might get a dangerous Taunt Minion out of the way). Seeing as this could theoretically be intentional behaviour, though, we could well see the card reworded in the future if and when cards are released that do exactly this. For the time being, it's safe to assume that cards which take control of a Minion only for one turn give that Minion Charge implicitly.

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.