Nothing wrong with a teen having an iPhone imo. It isn't even uncommon for teens to have iPhones and other expensive phones - most come via contracts and there is no actual big outlay for them. DD's iPhone cost about £60 on contract, possibly even less.
It is only on MN where teens are not allowed these kind of phones and tablets.
Your DD made a mistake. You say yourself that this is her first mishap and in the past has always proved herself sensible.
Accidents happen. I put mine in the washer, quite by accident. It was on the bed and I scooped it up with the bedding. Realised 10 minutes later - by that time it was obviously too late.
First, is it insured? If so, can you claim for a lost iPhone?
If not, then check that it definitely can't be retrieved any other way.
Else, get the quote for the radiator to be lifted off and replaced. You say that your DD earns money and has savings - can she not pay for it?
But I can't see the point in being furious over it. It was an accident, I doubt very much she even realised it was possible to get stuck - I would assume it would just fall down to the floor. It is DD who will currently be phone-less and no doubt upset about having it "lost" to her. And it will be her money that is needed to retrieve it.