Some comments and concussions:

• we archive everything in MAM with exclussion (chat states, etc)
• in 8.1.0 all offline messages are also included in MAM
• in 9.0.0 we disable 'offlinemessages' and swich it's handling to MAM completely
  • we would also include (in 9.0.0) XEP-0427: MAM Fastening Collation and XEP-0422: Message Fastening to optimise archive retrieval

MIX/MUC compat:

• we would/could require users to register (to avoid generating flood of participant member on MIX end from MUC users constant disconnections)
• we would/could add timeout for MUC users joins (couple/dozens of minutes) before generating participant event.

@wojtek Thank you for keeping a record of our discussion. I've created tasks and linked them to this issue for a reference.

As for MUC/MIX, I think that we will end up choosing one way and then maybe switching to a different solution as it is really hard to decide.