Thank you for your question.
You may have learned completely now that in meetings of that sort, NO ONE is YOUR friend. Speaking the legal truth that a Union Rep is "not necessary" for a meeting to proceed is also usually misleading: the u nion worker ALWAYS has the right to union representation under the terms of the collective bargaining agreement, federal law, and CA law...until that worker "waives" that right.
A true "waiver" is the "knowing and intelligent relinquishment of a known right." The first thing to do is bring this up with YOUR union leadership and enlist their help. What is "necessary" to management is not always the same as what the worker has a "right" to do or have.
What can be done? Well, the answer to that starts with the answer to the question of "What harm was done?" Your union rep(s) and/or a good local employment law attorney will be in the best position to find the harm, and of equal importance, whether having a union rep there would have made a difference. I must alert you that there are violations of rights every day all over the world which never go anywhere either because it did not really cause any harm, or didn't cause the harm the worker complains of (OTHER things made the harm happen), or the worker really, truly, knew that he or she was giving up important rights.
You talk about "revers[ing] what was done", which also makes me worry about time limits to raise an objection. People, including unions, are free under our Consititutionally-protected "freedom of contract" to agree to time limits that are different from what the law provides, so you will have to review what your rights are under YOUR union contract with the few people who are supposed to be on YOUR side--the union and maybe your own attorney *who can review the collective bargaining agreement* itself.