I think I got it, though it took a while ... for the relationship between forfeit transaction F and round transaction R, you want a *unilateral* constraint. That is, the server can get F only if R; and Alice (the participant from previous round) doesn't need to play a role in that round creation process (well apart from actually providing an F of course).

To have it work the "atomic swap" way it would have to be the counterparty (Alice) that was broadcasting the R, unlocking a secret for F. But that's not how this works, the action is all controlled by the Server.