I would think you have a few unrelated problems.
You need to fix the cat. I'd wonder if the stainless pipe finally gave way after 11 years. Cat'* don't just "go bad", usually a faulty O2 sensor richens the engine and they die from too much fuelling.
The traction control, you'll need to get it TechII scanned to find out what the true situation is with that. Although certain items can cause TC to become inoperative.
The transmission is slipping in fourth. This is usually caused by the 4th gear spline. Check the receipt for your rebuild and see if that was replaced, as well as all the clutches.
00 Regal GS: All kinds of cool stuff.