ping @Aljoscha, @Tim Makarios wants your attention on their comment: %oDD2oqV...

Surely the certificate (h7, h45, h0123, h01234567) proves only that the messages supposedly prior to m6 were created before the certificate, not necessarily before the message. Otherwise it would also prove that h7 was known before m6, which is backwards.

To prove that a message was created after certain other messages, I think the certificate has to be part of the message itself, contributing to its hash, doesn't it?

I'm using Manyverse, so I can't see your diagrams.