Does this sound like a good plan and does anyone have any advice on my rights in terms of having a disability, now becoming a carer and my ongoing mental health support needs. I basically want to be informed and ready should they try to make me feel so uncomfortable that I leave etc
You do need to be advised by your union, but also you need to bear in mind that nobody here, nor a rep from outside, knows the organisation like you do. In particular, you know better than anyone how they are likely to react. With that understanding....
I think you need to firstly undertand that having a disability, being a carer etc are vastly overrated when it comes to "rights". If you assume there are none, you won't be far off the mark - there are some but they are limited and often next to useless. In the end, the defining measure is how good the employer is at these things because the legal requirements are blurred around the edges, subject to many variables, and all have that word "reasonable" attached ("reasonable" being about as long as a piece of string).
I am also going to have to be blunt about this - as far as your manager appears to be concerned, the writing is writ in large letters on the wall. For better or for worse they will not be a force for your long-term future in this role. They want you out. And you know that. And if redeployment is a possibility from the employers point of view, think long and hard before rejecting it. You are going back to a scenario which will continue to impact adversely on you, and unless you have some confidence that raising a grievance would go your way and the manager would be dealt with, don't expect a lot to change.
That brings me to your list of complaints. If you are going to make a complaint about the way you have been treated, do not do it half-arsed. Make it a formal greivance, or don't go there. What you say is going to be shared with the manager anyway, but if all you are seen as doing is "whining" all those issues are blocked for further complaint. You can't give them a list of "this happened and it's not fair" and then expect to use that as ammunition again in the future. You want it formal and on the record. And if you aren't prepared to do that, be very cautious about how that makes you look.
You also need to look at this in relation to what you decide to say about the dynamic of the team and your struggles with that. Frame it around any grievance issues, but equally be aware that a long list of "everyone else is the problem, it's not me" is going to look very much like it is you! That might play in your favour if you would like redeployment - the more difficult it is to envisage you in that role/ team, the more likely they are to look at alternatives. But those alternatives may include paying you off - how would you feel about that?
Finally, deal with work and personal life in separate boxes. Get all the work stuff dealt with first, then have any discussion about caring / personal life. In the end, even with the best of employers, your personal life is not of any great interest beyond how it impacts on your work. They may dress it up in nice policies and empathy, but they really don't care all that much. So don't blur the edges - keep the two in their own respectve places. If you need some flexibility, then explain what you might need and it won't interfere with the work.
Two final points -
Are they really going to let you discuss all this with the Head of Department without having your line manager involved? Because frankly that isn't very good management. It's the mark of a toxic employer (rather than a toxic section) that such a thing would happen. Anywhere I have worked or seen, any complaints and discussions about work related matters go through your line manager unless you have already raised them and been unable to resolve them with your immediate line manager. Possibly, if the line manager is the subject of the complaint then yes, you may be able to go "up a notch" to make a grievance about their behaviour, but discussing team issues like phased returns and flexibility shouldn't be dealt with at this level. It undermines any management process if the managerial hierarchy isn't adhered to on matters of working practice.
Secondly, I am sure that your "list" of things you want to discuss are phrased here as simply that - nothing more than a list. Just be cautious though because they sound a little like a list of demands. Just be careful not to make them come across in your meeting like they are demands. That can irk even the most even-handed of managers, and you want them on side.