Are you saying that the bow is off 6 inches to the right or left of center? And the stern is off 18 inches off of center.

That a pretty strong distortion.

One of the most important aspects of your boat is that it runs in a straight line. If you turn the boat upside down and sight down the hull, is it straight? this is the most important thing. Did you try putting the forms back into the hull? That would be my first approach. Get the hull symmetrical.If you have to screw some cross braces of wood into the inside, do it.

Generally the deck is frosting on the cake. Is your cockpit riser and coaming done? Here's a desperation suggestion. You could cut the deck in two long wise and fit it to the hull. then add strips down the center to make the halves match which would look decorative. If the coaming is in place, you would cut up to them. I have never done the black plastic in the sun routine. But I have bent and pulled some hulls a few times almost to the point of cracking. Sorry to hear about your problem. its tough to diagnose with just a description. Heating might help? good luck.