Picture is good, Mic. I cannot guess why the PO did this, as I don't see any improvement to the backstay, the lazarette, or the tiller! Certainly that springy chain plate gives little support to the mast until the mast bends enough to straighten the chain plate. Over time, this constant flexing will likely cause a fatigue failure in the chain plate, which could cause damage to your mast. I think I would put it back in the original position.
AS for the trailer repair, there are several threads on this site detailing procedures for supporting the boat while you move the trailer out from under it. Try a search. Basically, you lower the tongue as low as possible, which elevates the stern. Build a supporting structure under the stern (structural wood frame, carefully stacked concrete blocks, or something else that is striong enough and will not tip over). A couple of your old tires on top might be good- block them so that they are paralllel to the hull to spread the load. Whatevr you do, be safe and make sure it will not collapse on you. Once the stern is supported, you can crank up the front jack to raise the bow high enough to clear the trailer. You can block up a beam under the bow, extending outside the wheels so you can pull the trailer out. Or, if you back hoe will handle it, you may be able to rig a sling to pick up the bow while you move the trailer. Just be careful. Some of the past threads have different methods and sometimes some pics. Good luck. -Paul