For the first question: I have a similar question that I don't see answered the other direction: If a UC member, will one flying on a B6 flight have access to UCs? For the second question since *G access is based on the operating carrier and B6 isn't joining *A I would be somewhat surprised (but not shocked) if any non-UA club access rules changed.

For my question: I have found myself flying somewhat frequently CLE-xxx-BOS on UA and VV lately. B6 operates CLE-BOS as a nonstop -- probably the only B6 route I'd actually have use for -- and if I can still make use of the UC I might actually at least try B6 for the next time (after things go live) that I need to wind up in Boston.