Your example is helpful, but I still believe you've drawn the wrong conclusion in the end. I fully intend to look at "values written for B".  It's completely irrelevant what data from A was written into B.  What's crucial is that I pick a sequence on B such that everything beneath that sequence was contained in A as of the last sequence observed by the client.  I think that's exactly what my version did.

Sorry for writing this after you rewrote everthing.