I would make the sauce in the roaster but do the noodles on the stovetop and then add them to sauce. Somehow, I don't see the roaster as a good pasta cooker. Cook the noodles to al dente then they can finish in the roaster. As for amounts, ya got me there. I would think if pasta dishes could be made well in the roaster, they would have included that in the manual.
I could give up chocolate but I'm no quitter!