L&D,
Hashi's advice is excellent and spot on. Don't let their attempts at game playing to bring you down, it's pure tactic on their behalf, and they need proof as Hashi mentioned. I'd go to the OH meeting too. The union can be a great help, so keep in touch with them and keep them in the loop.
If things get really bad, you can seek legal advice and request a copy of all your records held by the company under Freedom of Information with a short deadline for them. Don't tell the company you are planning to do this though, best to keep it as a surprise. You will then have all of your performance documents and I am sure you will find that they do not have a leg to stand on. They have not followed procedures. They have not given you necessary warnings, they have not offered opportunity through training to improve anything by the sounds of it.
OH and HR cannot make decisions on your health needs, only you, in consultation with your T can make these decisions. So don't let them pressure you, and you don't need to tell them your story. You are in control of you and your needs. Not them. And, they cannot force your hand to do anything you don't want and don't need.
Keep calm and cool when meeting with OH. Just listen and don't debate. Respond with non descriptive things like 'I will think about that' and 'thank you for your suggestions'. Remember that this is all the politicking of companies and not a reflection on you as a person. Anyone who has worked in a company, PTSD or not, has faced this 'nasty' and 'illogical' side of business. Don't let their poor management and their lack of competence in management get to you. Remember, you are strong, you are capable, and you are a survivor!
Please have a union rep with you for any meetings with OH, HR or whenever in doubt have them there. If anyone says you don't need union rep, they are lying. Tell them that your will book a meeting next week with union rep present.
At the OH meeting, ask questions as this is an information gathering opportunity and not a disciplinary service. Questions like why the company feels you need OH assistance? what can they do for you and what outcome are they looking for from these suggestions? You don't need to tell them anything about yourself as this is private information. Let OH know that you the support of Professionals outside of the company, and that you will discuss their suggestions with your Professional and get back to them.
It sounds like the manager has egg on her face as when you went away, she was not able to manage what you do every day at the company! Some serious lack of competency on her part and a nasty adult tantrum too! Shame on her! Don't let her issues become your own - they are her issues, not yours.
Hang in there and let us know how you get on. You are a survivor and of great value. Remember that. The only other thing I would do is to keep a paper trail as much as you can, and do keep the union involved every step of the way. You have rights.