You are aware that the union may refuse to represent you? Most unions have rules that will not allow them to represent you on a pre-existing issue or with a certain period of joining. You may get a local lay rep if one agrees, but there is no legal or full time officer support available except in very specific circumstances. Union membership is like insurance - no real point in having it after the event, you are supposed to take it out before your house burns down.
I am a bit perplexed by the fact that they didn't continue the process immediately if you failed, and equally by the "informal" comment - it does sound very much like they have made a process error and therefore can't immeditely proceed. That might buy you some time - if this was "informal" then they need to go to stage 1, nor stage 2 (you need to check the policy but if there is no "informal" stage then you shouldn't go straight to stage 2).
The other thing that occurs to me is that they may have decided to think about offering you a settlement anyway - especially if they do have to go back to stage 1, since all of this is going to end up a lengthy process if you are correct and they want you to go.
On the positive side - although there don't appear to be any indications yet of this, if there is a restructure in the offing perhaps they have in mind a place to move you?
I am sure not everyone will like me saying this but I will say it anyway. Of course HR are validating your manager. HR are paid for by the employer. Their job is to protect the employer, and to get the employer to where the employer wants to be. If that is where the employee wants to be too, then that is a happy coincidence. But they are not and never will be on your side. They are not neutral and they are not arbiters.