Robin,
Think of the two plates as splints if you like, but the plates and bolts are really operating as a lever. You know how you can lift a heavy object rather easily by using a long board with something 'jammed' underneath it close to the object your lifting. Well, you can also stop heavy things from moving using the same priciple. In this case the heavy object is the load of the mast step and we want to stop it. The plates work in conjunction with the bolts to form levers and fulcrum.
I don't have the manual memorized yet so I can't recall if it asks you to relax the standing rigging slightly before jacking. But if the mast has been stepped and unstepped several times and if the standing rigging has been adjusted or tensioned in accordance with the changing (lower) mast step you may have to relax it before jacking. I, personally, would jack the beam before starting to work on it. It's not realistic to think you can take all of the sag out of the beam by jacking it up. However, restoring the cabin top to its original shape or very close to it would be very desirable for an afternoon's work. Without a doubt, jacking up the beam against fully tensioned standing rigging or lifting the cabin top beyond its original position would not be good.
The three bolts allready there? No idea. As sencondary and tertiary owners we sometimes run across some strange @*&#!
Careful on removing that laminate! That's exactly how our redo got started. If your heart's set on it try removing all the trim that's necessary first then lay a piece flat metal stock (like a framing square) on the cabinet top or face to protect it and provide a straight edge. Then score the laminate repeatedly with a utility knife. Instead of going for broke in one pass with the knife try eight or nine consistent passes. Heat the laminate with a heat gun or one furious hair dryer and slide a 6" putty knife behind the laminate to work it loose. Then try to 'snap' it off along the score. Work in straight lines. I, more than others, understand the need to snoop but keep in mind you'll need to put something back. Know when to say when! Most of all really study what you find. For what it's worth, Tony G