Dang plugged exhaust will get you everytime.
Got to look in both ends when living somewhere that has hole plugging critters. Maybe leave the baffle out and ride it hard for a bit to burn/purge whatever's left.
Ugh; sanding plastics. That's the best option if you can achieve a reasonable finish. You have to take the time to blend all the scratches while switching to finer sandpaper or have to start again. If there is crazing that's too deep to completely sand out, I don't hesitate to paint them. Properly applied paint will hold up very well and it protects the plastic from further sun damage.
You've probably had some old plastics that someone spray painted decades ago and found out just how tenacious that can be when trying to remove. It's like after enough time, the chemical bond is just as strong as if it were molded in.
Saving the OE plastics is worth the effort though. Nothing fits or looks like they do.
The story of three wheels and a man...