Two-Tone Layering Question

I'd do the black last. As for painting the stripe, black will cover the purple in one coat, two at the most. Due to the thin coat(s) of paint, there shouldn't a huge ridge where the tape was, plus once the paint is cured you'll be polishing it, and that alone can minimize the ridges, if any. The only way you'll never have ridges is if you did the whole car in base/clear and cleared over everything.

Personally, with the quality of the vinyl stripe kits these days, I see nothing wrong with the decal stripes.