All of this has already played out before, some ~20 years ago. Microsoft wanted MSN Messenger to be compatible with AOL's Instant Messenger, so they reverse-engineered the protocol, only for AOL to update it, breaking the compatibility. It went back and forth until Microsoft revealed that AOL was using a buffer overrun exploit in their client to do remote code injection in order to authenticate the client.

Apple will never allow Beeper to exist; there's no point investing any time or money into it as whenever they manage to sneak back in, Apple will boot them back out. Perhaps some sort of legislation will fall in place forcing Apple's platform open, but given that they're implementing RCS I somehow doubt it.

Further, we know nothing about Beeper as a platform. It can/could speak with iMessages, but then what? How do we know it's secure? Because the owner of the product says so?

If the idea is to get secure and encrypted messaging between an iOS and an Android user, why not go for something like Signal that's open source?