Talk to an employment lawyer. If you really did break the rules I doubt you could get reinstated, and from the sounds of it, I doubt you would WANT to be reinstated. You have (and it sounds like, for some time, have had) no future in that organization. Holding onto your job via purely procedural/legal arguments might be/have-been possible but really there's no chance you'd get a promotion or any sort of interesting advancement. If there's no chance for you to grow in that job, why on earth would you want to be there?
I agree your union organizing activities likely contributed to your downfall. If I were your boss, I'd probabably be out to get you if you were militating for a union as well. I'd try my best to "do it legal", but I would find a way to get you. To be honest it sounds like you've had kind of an "us versus them" hostile attitude towards your former employer, and perhaps other supervisors do not come across as having the attitude problem that your boss perceived that you had.
So, really, the question you should be taking to the labour lawyer is whether or not you got an adequate amount of severance, or whether by going to court you could get more.
The labour board is probably not the right venue for this, but a labour lawyer will be able to tell you that. I'd guess it's not the right venue because you got caught with your hand in the cookie jar. That other people had their hands in the cookie jar won't matter to the board--a complicated argument about discrimination probably needs a court.
Anyway, a labour lawyer should be willing to extend a free telephone conversation to you where they'll spend 20-30min listening to your situation, at which point they will tell you what your legal options are and an estimate of your chances for success and what you might be able to get out of it. It won't cost you a cent, and it'll be better advice than you'll get for the same price here on terb.
Good lawyers won't just try and convince you to use their services either. I had a very different sort of dispute with a former employer once and went to a labour lawyer for a free consultation. To my surprise, the guy recommended a labour board approach in my case, which cost me nothing and earned him nothing, and that turned out to be very good advice as I got 95% of what I asked for. A good lawyer will give you the best advice he can, confident that enough cases will come along that he can make money on that he doesn't need to chase ambulences or lie to drum up more clients.