Anyone can already buy those certificates - but as its an artificial market where rules can be changed politically it's actually way more resistant to such things than regular markets, so if those hedge funds feel like they want to lose some billions they can certainly do that. There is a large enough stockpile of certificates + leeway when to submit them that any short term market squeeze will just be dealt with politically.

This argument, namely that politics can lower the price (by emitting extra certificates) when it gets too high, contradicts the whole reason for the mechanism in the first place: They claim a free market can find the right price better than politicians. But then they interfer anyway?

The price will rise much larger than a dumb, fixed increase-schedule would. Because the "market" wants it's profit.