It seems clear to me that Unite are trying to hold an "unchallengeable' ballot by providing those membership numbers. I think it's a perfectly valid attempt to get things right this time, and most reasonable people would accept that it's impossible to get it 100% correct when the Union's membership is constantly changing [both up and down, although probably more of the latter].
As to the inclusion of MF crew, I think again this is a fair move. The fact that IA has not been an issue there should not be a reason for excluding BASSA/Unite members on MF from participating in the ballot. How would people comment if ballot papers were only sent to those who had already supported IA? IMO this has to be a 'global' ballot if it is to have any credibility.
The crunch point is surely the list of "Items of Dispute". Doesn't that list represent a list of previous demands, and thus create a 'continuation dispute'? However, I can only assume that Unite and their legal people have considered that aspect ... and determined it is safe to go ahead with what may well be, for striking CC, 'unprotected IA'.
My money would be on the BA Legal Team being more competent when the inevitable Court case ensues.