My main concern with the protocol design is in the identity, and how it lends itself to centralized name services (@alice.google.com). The end result seems like it will be no different than Mastodon and email protocol, where a few central players own the majority of the namespace and network effects prevent most users from having full custody & portability of their accounts.
A permissionless blockchain does seem like it would be a suitable solution, but your identity page mentions avoiding blockchain due to slow commitment times:
> At present, none of the DID methods meet our standards fully. Many existing DID networks are permissionless blockchains which achieve the above goals but with relatively poor latency (ION takes roughly 20 minutes for commitment finality)
What latency is really needed for a global name registry of a social network? I've only registered a few Twitter handles in a 10 year period, and each time I would be OK waiting for 15-20 minutes (or longer) to ensure commitment finality if it means I could escape the centralized host at any point in the next several years. Similar story with rotating keys and updating any pointers to my host/server.