Understood this part, using HTLCs is a brilliant idea. Just can't figure out this one edge case in my head:
Now Bob is able to retrieve the X BTC using the secret R. And as soon as he does, the secret will become available to Alice as well.
What if Bob doesn't share the secret right away, but waits until Alice's channel with me expires (our channel with him is set up for a longer period of time), I take away all the funds in the channel with Alice (she can't do anything, because she doesn't know
R), and then Bob publishes tx with
R when it's too late for her but not too late for him, thus he also takes away funds from Alice in his channel? We've successfully defrauded Alice, haven't we?
The 'Broken channel'- and 'Rerouting'-part should clear your concerns.
Also very clever, thanks for the reference, doesn't address this case though, if I understood correctly.
In 'Broken channel' paragraph there's an assumption that the secret has been voluntarily given up by the last participant.
In 'Rerouting', it is assumed that the immediate neighbor cooperates and creates a negating transaction, rebalancing the channels.