I think implementing just the e2ee part of the WhatsApp protocol (which happens to be the Signal protocol with open-source libraries available) client-side and have a server-side bridge transport the encrypted messages over to WhatsApp and vice-versa is a sensible option not mentioned in that blog post. Yes, worst-case it means that for interoperability you have to create a bunch of message encryption routines. But we are effectively talking about iMessage and WhatsApp - Facebook Messenger doesn't do e2ee and no other company that built a widely used personal IM system is big enough to be covered by DMA.
Regarding moderation and spam: I think a company with €7.5B yearly revenue should be reasonable able to build something such that moderation and spam prevention are also possible with federation. Google already does a pretty decent jobs in spam filtering with e-Mails, I guess they should be able to do something similar with IM.
Regarding moderation and spam: I think a company with €7.5B yearly revenue should be reasonable able to build something such that moderation and spam prevention are also possible with federation. Google already does a pretty decent jobs in spam filtering with e-Mails, I guess they should be able to do something similar with IM.