Minimal signing flow changes for TRUC channels

Again may be turned around here, but I’m unclear how this helps with the specific problem.

Going back to the running scenario posed by @morehouse , Bob has already forwarded an incoming HTLC that is yet un-resolved. So what we’re concerned about here is Alice’s commitment transaction having a valid signature from Bob, but no way for Bob to claim the second stage HTLC-Success transaction (which would now have to be pre-signed for anti-pin reasons). How would adding a signature on an old commitment transaction resolve this? Alice can still immediately go to chain with the new commitment transaction that doesn’t have a signed second-stage HTLC transaction, and now Bob has no way of resolving it in the success path.