If I “go live”, I lose the ability to go directly to the *.platformsh.site url. Can I go back to using that at any point?
You can add it explicitly as a route in routes.yaml in addition to the “{default}” route(s).
Bear in mind that having the site publicly available on development URLs (or multiple URLs simultaneously) can pollute SEO or analytics results. Normally a redirect to one canonic domain would be a better idea.