In order for the first part to work, they'd need an API to access the RCS network. Today, there's no API that gives a third party access to the core network, which would then treat Google as a dumb pipe - meaning only an encrypted message would pass through them... utterly useless to Google's core business.
As far as the app, that'd be specifically on iOS - so no alterations to their app at all since it'd be purpose built. Google could launch it today and have it as RCS-only. RCS is only an over-the-top instant message service, it requires no core carrier network activity - it's how they circumvented carriers entirely on this deployment in the first place, who up until this had their own closed-network systems that only communicated an RCS message with other users on the same network.
Really, RCS is no different than any instant messaging app; WhatsApp, Signal, Facebook Messenger, and so on. Their goal here is just to be the default app that comes on every phone, everywhere, as in spite of the dominating presence of a WhatsApp it still isn't on 100% of smartphones... but the Messages app your phone comes with is on 100% of smartphones...