However, I don't know if this attack makes any sense - could the channel partner access these funds or could they only be mobilized again if both channel partners cooperate and provide their signatures to close the channel?
If you hand me an address, and i do a transaction which pays to another address. Would you accept it as a payment ? No.
If we collaborate to create a transaction to pay you, and i finally broadcast a different transaction, would you accept it as a payment ? You should not. This is not an attack, just an absence of payment.
This is why I think the proof of payment feature of Bitcoin Lightning Network payments is important, and that we *must preserve it*. We can always bikeshed on the definition of an onchain Bitcoin payment, and endlessly argue if there was a transfer of value. If we use the Lightning Network, we just have a proof that the transfer occurred.