Here are my thoughts. I haven't worked with Tyvek myself but I have been reading about it.
Tyvek isn't 100% waterproof, so maybe not the best tarp material for the weight. It will take rain for a while, but taking a zero day because it's raining might not work out. Sewing Tyvek is ok, as long as you use a low stitch count (6-8). You might try stitching the cat ridgeline, then felling the seam with the tape. Should be best of both worlds.
Good luck with the project! Post pix when you're done, if possible.
Edit: Apparently I was wrong about Tyvek's waterproofness. I was going by what I had read about the TT Sublite.