Since you haven't provided your real domains, example pages, or screenshots, we can't review your work.
The method you've described, as long as you're not trying to redirect assets other than HTML pages (i.e. PDFs), is workable. If a URL appears as both a Redirect Rule "Original URL" and as a live LP URL on the same instance, the redirect takes priority (which is what you want).
So you have to give us more concrete examples of what you've tried.
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