Minor clarification: oposts reside on PDSes, so they're still not ephemeral with non-archival relays.
I'm not sure about relays talking to each other, it might happen more at the appview level -- appviews talking to other appviews, appviews listening to multiple relays. There are various projects looking at independent and semi-independent subnetworks, I'm not sure what architectures they're experimeting with ... It's hard to predict at this point what will and won't work.