any solution that enables extra proprietary functionality outside of the standard would be non-compliant, so I don't think they will be able to get a half-baked solution through.

Again, there is no particular standard being mandated for this. The EU just wants interoperability – being able to communicate to a person using iMessage on their phone from your phone using Whatsapp for example. How the providers accomplish it is their business. The only pressure will be to not degrade the user's experience to the point the interop becomes a liability rather than a boon.

the EU has zero interest to play softball with these companies. If they can punish them with a billion euro fine for not complying, they definitely will.

OK but they cannot simply slap fines on them and call it a day. If the companies say "what you're asking for is too difficult" they will need to take the time to examine this claim. In fact I can guarantee right now that we'll see the deadline extended at least once.