I don't have the same frame as yours, but it's a Felt Z frameset of similar vintage. It has what appears to be the same seat tube construction as yours.

I can tell you that the clamp definitely clamps onto the insert, not the carbon fiber seat post. It doesn't touch the CF, except resting on the top lip of the tube.

There is a metal insert inside the CF seat tube like yours. Mine's black anodized, but same principle. I call it an insert, not a shim - it is structural, not just taking up space as a shim, as far as I can tell. Yes, it reduces the ID of the seat tube to 27.5, but it provides the metal structure to clamp the seat post instead of the CF.

The metal insert protrudes about 1 cm above the seat tube. I haven't put force onto the metal insert (only hand strength on the insert itself, just now), but it doesn't slip to rotate or up and down. I've ridden the bike for over 10 years and the saddle has never twisted on it's own, nor have I tried to twist it. It certainly doesn't slip up or down as long as the clamp is torqued properly and I use assembly paste.

Both the metal insert and the carbon fiber seat tube have slits. The seat tube slit is at the rear, and one of the insert's slit lines up with it.



There is a thin metal shim that is around the protruding part of the metal insert. It also has a slit. The seat post clamp strictly clamps on the protruding part of the insert (via the shim)



The seat post clamp has an inner lip which prevents it from slipping over the carbon fiber seat tube. It clamps on the metal insert only (via the shim). I don't know why the carbon fiber seat tube has that slit, maybe so it can flex a little under clamping because the metal insert is bonded so well that when it flexes under clamping, it flexes the CF seat tube with it.



As a comment that does nothing to assist the OP's current situation - if I discovered that my insert slipped with super-minor force, i.e. not heaving on the saddle to see if I could twist it, I'd have probably pulled it out as far as it came with easy hand strength, smeared some epoxy on it, and put it back into place, assuming it was limited to how far it goes in, leaving the ~cm protrusion. Or I would have called Felt or contacted a trusted local bike mechanic to look at it and advise. But, I really doubt there's a lot of twisting force the insert is subjected to so my inclination is to think it would have been easy to just bond it a little bit to solve the problem.

At this point, I'd re-insert it with ample epoxy. I don't know what I'd do about the crack except maybe stop it with a tiny drill hole and include hope that by epoxying the insert into the tube, it will "splint" the crack, eliminating additional cracking. OTOH, again, stopping the crack, I think it would be easy, if not pretty, to repair externally, looking up CF repair online.