Sorry this is a little late but that little aluminum casting that should rest between the mast post and the dolphin striker is to keep the mast post centered on the dolphin striker - to keep it from slipping off to the side as just happened. The dolphin striker (truss) should carry most of the load from the mast and as Andrew said, mast post tension should be adjusted to get a little prebend in the main beam. This ensures that the dolphin striker is carrying the majority of the mast load.

You're right that there should be a compression sleeve (although it's not terribly thick) inside the beam. If the beam had a flat top, I would consider putting a plate on top to provide structure over the hole. However, this would not work very well on the round beam. You could, however, go to the trouble of forming a piece of 1/8" to 3/16" aluminum plate to fit closely around the curve of the beam and restructure it that way - but a replacement beam would be less trouble.