This sounds at least superficially reasonable. I'll think about it more tomorrow.

Any more comments? I’m thinking about implementing this soon

I realize there's a tension/inconsistency between different points of your proposal. As in your point 3, I believe that readiness to handle messages in an epoch needs to be on a per-object. I might possibly extend that to a per-handler basis. That conflicts with the notion in points 1-2 that this is a per-epoch notion, seemingly across all objects.

More broadly, this in some ways resembles notions in more dogmatic Actor programming models of objects having a 'mailbox' from which they select inbound messages. We're definitely distinguishing our approach in that control of mailbox-processing-order is living at least partially outside the recipient, but we should keep that in mind.

The “dependent” bit can never be unset for a given epoch. It just puts the epoch in the category so it can be treated accordingly by the runtime.

The lack of readiness is handled by each component in VT. For collections, each element can track its readiness for a given epoch individually.

Had a thought earlier today, which may or may not be a productive direction:

The chain manager can arrange things so that none of the messages in a chain actually get delivered until it's got the whole chain defined (by not calling finishedEpoch on the dummy initial epoch). That means that it can send a control message describing the whole sequence of upcoming epochs to each object, before it would start executing any of them. Perhaps this makes the problem easier to solve, but with the proviso that the generation of the sequence could be constrained by how we might implement this.