The union or indeed any pilot can nominate anyone to act as their bargaining representative - they don't need to be elected. In other unions/workplaces much of the negotiation can be carried out by hired negotiators.
There is nothing "legally binding" about a deal made in the room - it just determines what gets put out to be voted upon by the pilots. The negotiators cannot "make an agreement" - they just come to an agreement "in principle". The agreement is made when a vote is completed in favour of the proposal. The company can choose to put a deal out for vote even if the negotiators do not agree/support the proposal.
I'm not going to say anything here to push the SSA pilots to a certain view - as has been said by others you should properly consider the document that will be put before you. But also realise that no EBA exists in a vacuum - what happens in QLD will affect the industry as a whole and vice versa. There is a lot of information around EAA circles at the moment which examines the financial impact of various proposals - I would want to know that any agreement put before me for my vote was supported by bargaining reps who had a high level of understanding of what they were negotiating. All I would say to my SSA friends is that they should read and understand the document they vote on, and take the opportunity to question their reps on how they determined its worth.
Nuff said from me.