If people from Mint are saying it’s on their end why wouldn’t you believe them?

You’re not owed the technical details. If you want to encourage/pressure them, contact Mint support. They’ve already said that it’s a priority.

You can use RCS to message anyone on a carrier that supports it for their device from a carrier that supports it as long as you both have it enabled. There is a setting on your phone to turn it on. Consult the documentation for your carrier/device.