I agree with Sgt. Rock. The laminate is polyester, so it doesn't stretch the way nylon would, but the thing that limits diagonal stretch the most is the fact that the dyneema fibers are held in place by the outer layers - they can't slide past each other the way fibers in a woven fabric would.
Making a cat cut ridgeline is probably very tricky. I haven't attempted it because I don't think it's necessary. Getting "an extra yard or two" is going to cost you another $50. I suggest a plain ridge seam for the tarp with a ridgeline below the tarp as part of your suspension.
You're asking interesting questions, which shows that you're thinking. Keep thinking.
people who make catenary ridgelines with CTF3.)