If I remember correctly, one of the reasons CA chose to be in *A is that the PRC government CA should join the largest alliance which just happens to be *A
IMO CX is less likely to veto CZ than MU to join oneworld if CZ does apply for membership especially after the jetstar hk attempt. They might in future but I'm not entirely sure if that would happen in the near future, since Xiamen airlines which CZ holds 55% has stated that they have no intention to leave skyteam at the moment and it would be most logical the 2 in the same alliance. On the other hand I don't think CX will join *A in near future either with every member in *A have veto power unlike oneworld, and I can't see why SQ/TG/BR/OZ not using it.
With the cross-shareholding I think CX continuous cooperation is quite certain. What would be interesting to see is that if other oneworld airlines (especially QF/JL) would switch from partnering with MU to CZ for Chinese market. Don't forget JL has only just submitted JV application with MU in summer. Forming partnership when there is no alliance partner in that country is one thing, but with another member in the same alliance but still choosing someone else is completely different. If CZ is really interested in joining oneworld soon, I doubt JL doesn't know about it when they are setting up the JV with MU.