As far as I understand: the expiration of a single HTLC in a payment channel with an unresponsive partner causes the whole payment channel to force close (Behaviour A). This has been discussed, for example, in this question; the addition of covenants to bitcoin would allow a participant to only settle on-chain the expired HTLC, without the need to close the entire channel (Behaviour B).
First of all, is my understanding correct? If it is, then why is not possible today to implement, using pre-signed transactions, the behaviour B?
I would like to understand what is preventing parties to have multiple pre-signed transactions to settle a channel on-chain, one that closes the whole channel (just as it is today), and another one that keeps the channel open (i.e. it has a 2-of-2 multisig output) and just settles the HTLC on the blockchain. Is it just technically difficult or theoretically impossible without covenants?
