It appears that the compression loads are being borne by the aft cockpit bulkhead and mast box rather than being properly transferred down the compression posts. As you point out, the compression posts look solid in the engine room but how about at the top where it interfaces with the box? I am not familiar with the all of the construction but check carefully to see if there might be some bad coring at the bottom of the box that is compressing. With this much movement you should be able to see something... Let me know what you find.
I will be back to my boat in about a week and will see if I can figure out what might be going wrong on your boat.
Personally I would get as much of the compression load off of the mizzen step as you can until repairs can be made. The fibreglass will eventually take a set and at lest some of the deformation will become permanent. The fractured fibreglass will need to be rebuilt to restore the lost strength.
SV Sueno, Maramu #220