MMB is buggy especially when it comes to details such as the baggage allowances on mixed class bookings.

I really do not think 30 kg limit will be honoured. The baggage limit is determined by the operating carrier and I remember seeing something to that effect on BA.com as well. I strongly suspect showing BA MMB page to CX saying 30 kg won't do any good at all, unless you happen to come across an exceptionally kind supervisor (probably can't be authorised by a standard check-in agent).

If it was BA, they might be cooperative since it's their MMB bug, but it's not BA, so I wouldn't risk it, particularly as CX excess baggage charges are pretty expensive.