@cft

[...] which seems to have been overlooked.

Nah, just discussed elsewhere.

With a cypherlink-only approach, the query interface in createHistoryStream would have to change.

Can you elaborate on this? createHistoryStream only returns messages from a single feed, and on those backlinks give us an total order (irreflexive), with or without any sequence numbers. So I don't see how createHistoryStream would be affected at all by removing sequence numbers.
Note that createHistoryStream is used for (legacy, non-ebt) replication, not primarily as an API for clients.

get up to N causally-youngest log entries (the heads so to speak). Ideally only one entry exists, but better prepare for tangles where there can be multiple tips.

Ssb forbids those. A forked feed does not get replicated at all.