Question - If crew turn up and operate on the 16th and thereafter, will they be deemed to have accepted the new crewing levels? I take it that anyone that refuses to operate will be suspended - how many will do that if there is no guarantee of support for a strike?
[flippant]Ask BASSA, that's what they're there for.[/flippant]
Unless there has been a legal ballot in favour of industrial action on 16th (which there hasn't) and appropriate notice has been given by BASSA (which there hasn't), you will be in breach of your contract if you fail to turn up for a rostered duty and will risk disciplinary action. Beware "pattern sickness" in that regard as well.
As to the first part of your question and bearing in mind I'm not a lawyer, in the long run the courts would probably suggest that as crew have been operating to the new levels for x weeks or perhaps that no ballot had been held in advance of the changes, they have accepted them. It would be for BASSA to come up with a legal reason why they should revert back to the old levels, which neatly brings in "Some Other Substantial Reason".
Sadly, your union has been caught by the short and curlies.
I have a question in return:
BASSA said they would ballot immediately if BA announced imposition. They haven't. Why not?