That is not clear yet, since the law does not specify the protocol needed to make that happen. The law forces them to make the messenger interoperable while keeping features like E2E encryption. Basically like ActivitPub with more security / privacy features.
No they do believe that, because in Europe that is kinda true. You might find someone who uses iMessage if you go to the UK, but in mainland Europe iMessage is the same as GoogleMessages it is „the SMS app“. In modern days it is receiving messages only for most people.
FaceTime is another app that is hugely popular in the US but has almost no users in Europe.
But in the end it is installed on all iPhones so they will fall under the rule, which is not a bad thing.
How would that be a feasible solution?
If they stay true to their word, they will force them to implement interoperability and not just force them to give some sort of API access.
So if you install WhatsApp their plan is that Meta must provide a seamless experience in talking to every other person even if that person is using e.g. iMessage
A new protocol for the given reason is faster implemented, more secure and better scalable then just trying to do something with existing solutions that were never designed with something like this in mind.