Clean the joints around the knee on starboard side really good and inspect it with a lamp and loupe. Search for cracks. If there are no cracks, the hull
must have deformed a bit to allow the deck to come up. Check the deck-to-hull joint in that case. I suspect you will find no cracks and that the deck will not come down after releasing the tension. As it's only a little, I don't think the deck to hull joint will be affected either.
What I would do is keep the tension of the stays/shrouds for a couple of weeks and see what happens. In the mean time, work on the starboard side, removing the chain plate
(tie the capshroud to the toe-rail instead) and knee and clean everything up. I would install a stringer under the deck over a 5' or 6' length with the center where the chainplate is. The stringer will be between the knee and the deck but just to the inside of the chain plate (so not between hull and chainplate). You make a precise cut-out in the knee for the stringer. This will distribute the load over a much larger area, solving the problem. If the deck comes back to original shape after a couple of weeks, so much the better. If it doesn't, don't worry too much when you find the hull to deck joint in good condition.
The stringer is very easy. Start with a 6' long strip of 2" wide and 3/4" thick top quality plywood. Now fit it under the deck (with a cut edge of the strip against the deck, not flat!) and look for gaps between stringer and deck. If there are (there will be), do this: mark the position of the stringer and strut it in place without too much force. Now, transfer the shape onto the stringer by placing a pencil against the deck with the point touching the stringer and moving it along the side of the stringer. If the gaps are too big, place something between pencil and deck to just bridge the largest gap. If the gap is really big, you need to start with a wider strip so that the minimum width of the stringer after cutting the shape into it stays well above 1.5".
Taper both ends from 2" wide to nothing over a 1' length. Round the under-side with plane/sander/router whatever you have available. Give it the same treatment as the knees (impregnate with epoxy). Bond it in place with fillets (not too much pressure from struts but not so little that it can move) and cover with cloth-roving-cloth layers that spread out under the deck for 4-6 inches on each side and end of the stringer. The taper will make this easy. Cut the glass round at the ends for better stress distribution and overlap with each next layer bigger than the previous one. If the stringer is in sight you can make the last layer neat by cutting away frays etc. Wait until the epoxy cures but is still sticky. Put plastic food
wrap over it and use a metal ruler and razor knife. If the plastic wrap won't come off without damaging the epoxy, leave it until fully cured. You can pull it off then and use a little help from sandpaper where it sticks. Sand the edge so that it fairs onto the deck and finish it with more epoxy/fairing and painting (after the knee is fitted!).
On to the balsa core
. There shouldn't be any balsa where the chain plate goes through the deck. Check this, it should be solid laminate there. If there's balsa within an inch of the chainplate, this should be removed. This is easy when done from the top. Start with making a template out of carton or 1/8" plywood: nice round radius corners and 1 to 1.5" around the chainplate position. Make a cut-out in it like if the chainplate would go through. Now, completely tape the area on deck so that it doesn't get messed up (use overlapping blue masking tape, the widest you have) and cut the slit for the chainplate in the tape with a knife. Put your template on top with the chainplate through template and deck to keep it in place and use a permanent marker to mark the template on the tape. Now, you need to cut through the upper fiberglass
layer of the deck. There's many ways to do this. A router is best and in that case you make a different template which is a guide for the router. You can also use a Rotozip tool or even a jigsaw (tape plywood under the foot to limit the depth
it cuts). You can even use a Dremel with a cutting disc, but try to make it as neat as you can. After that, chisel & sand out the balsa core
. Remove some core from between the 2 glass layers around the cutout. I always use my Dremel with big drum-sander for that. Tape the hole for the chainplate underneath.
Now, you can start casting a ketchup consistency of epoxy with high density filler in layers of maximum 3/8" thick. Start with thinner layers and check if it doesn't overheat before going for thicker layers. Cast the next layer when the previous one starts cooling
down. When you get to the top, make a thicker mix so you can use a spreader to fair it nicely to the masking tape.
After cure you can cut the hole for the chainplate again (router, rotozip or even drill holes and file then out). It should be painted to protect the epoxy from UV exposure. If you get into epoxy like I did, you'll even get some white pigment epoxy additive to mix with the top layer for additional UV protection and easier painting.