Hello Cian, Welcome. Sorry for this long delay. Sometimes life can interfere with things. I see 2 issues here. They are separate. 1) The cross beam crushing: Please see Bryon Craig's thread on this issue titled "Mast Beam Crushing". It will be very informative, This fix is doable with the hardest part being the removal and re-installation of the headliner. Also, this is best done with the mast down. 2) The deck deflection: I don't know the reason for this on your boat. I have seen this exact condition on other boats. I could not live with it and would have to correct it but it will require "seriousness" and skill. But it is quite doable. Whether rotted or not in that area, it should not have deflected. Obviously the deck is not supported underneath in that area except by the mast support cross piece. A good, and strong cross piece, as shown by Bryon Craig, should prevent that from ever happening. Most likely, just jacking it up and adding a new cross piece will not work. I don't know and would have to inspect it and investigate with at least one core sample.
A new cross piece and perhaps a mast support beam should be made first to help establish the correct curve of the cabin top. Almost certainly, the deck skin will need replacing in order to conform to the new, proper, shape. it is 1/4" thick. There is so much more to say but this is not really the place to do it. Where is the boat? Let us know your progress. Good luck, Dave