The mast step seems to be in very good shape.
The cracks in the deck under it don't look so good. Gotta be a rotten deck.
On the inside, I really don't think your problem has anything to do with the compression post itself. You should be able to chisel out the old wedge-shaped pad at the top of the post and pound a new one into place.
Then re-core the deck leaving the bottom skin in place.
In any event, I took another look at my cabin sole. It looks like there was a seperate section of flooring installed around the post (between the two bilge hatches) about 4 "planks" wide.
This might explain how they installed the post with the deck on.