They mandates a bunch of things, more than just payment services. It also includes app stores, at least according to multiple reports, but linking to the specific article that includes app store freedom seems hard to me.
No. Some very large companies have to offer interop for their messageing platforms, smaller ones don't. So if Threema wants interop with Signal, Signal can say yes, no or they can discuss it. If either Threema or Signal want to interop with Whatsapp, Facebook has to offer that, because Facebook meets one of the two size criteria.
Part of me wishes that they enable it earlier, but the other part of me thinks Apple will lobby for 2 years to rally MEPs against it.
There's laws for messaging and app stores as well.