Rocket Science
I think you have inadvertently hit the nail on the head; as soon as all 3 sections of union agreed to go down the route of individual negotiation (and all 3 did agree to do so), then equal pay offers were off the cards.
As soon as it came to negotiate anything other than core pay, then of course the company is going to put different values on different aspects of duty.
Asking ATCOs to vote 'NO' to their individual pay offer to support ATSAs or ATCEs is actually tantamount to secondary action.