Hi Jeff,
The max shroud tension is determined by max righting moment. If you don't change max righting moment, you won't change max shroud tension or max chainplate load. If you add trapeze, the extra righting moment goes directly from the trapeze belt/person to the mast/rig/sails and does not go through the standing rigging. That is part of the beauty of the trapeze. The trapeze adds extra compressive loads coming down the mast but it does not add to max shroud tension.
As far as stepping the mast on the deck goes: This is the way to do it. The ARC mast step ball holds the mast base while stepping and unstepping the mast as well as being the mast rotation bearing. Take a piece of your broken mast and make a short column from the old mast step to the underside of the deck at the new mast step. This way when you step the new mast on the deck, the boat won't know the difference and all the loads on the structural system will follow the same paths they always have; no difference.
I don't know exactly where the clew of the mainsail would come to relative to the transom of your boat, but if it is close to the transom, you could beef up/make new and stronger the cross beam that supports the upper rudder pintle and install a mainsail traveller track there.
One of the first things you need to do is make a good estimate of the price of all of these parts. I would estimate you are talking about you spending 10k on parts and sails with you doing all the work. If you are not handy with doing this type work, add another 5k. You know how boat costs are, things could run up to 20k with a few surprise problems.
Good Sailing,
Bill