In my (limited) experience of this then a phased return to work in another department sounds like a good way forward. It gets you back into working, gives you some important income and confidence, demonstrates your willingness to return to work, and ultimately will prove your ability to be an effective employee (in the right supportive environment). Is there anyone who can help to advocate on your behalf to broker such an arrangement?
It is also important to be clear about the facts of the situation: so if stress was caused by x situation at work, be clear about that and how it made you feel. However if there were also other things going on in your life that were causing stress, it could weaken your case if you attribute everything to work. Is there some way in which you can separate the elements and their consequences? e.g. "my mother was ill, so I could not attend x meeting" and "my manager said [blah] and it made me feel y".
On your appeal, I am reluctant to answer because I am not an HR professional, but I would have thought that you should reiterate the facts. e.g You should assert that you did tell you manager about the stress, and that you asked for something to be done, before and after the event. I would avoid bringing in anything new at this point.
I would think that if you want to complain about the time to investigate the grievance process then this would be a new and separate grievance. Avoid confusing the 2 issues. You need have the facts placed on record, but what would you achieve from instigating a grievance at this point?
As the other post says - are you clear about what you really want? Are you appealing to strengthen your case for redeployment? Or are you making an appeal to record your position in the event that things do not improve? Are you seeking a change in policy or procedure that would prevent a similar issue arising? There are many possible reasons.
You probably feel very wronged and hurt by this whole episode, and there are things that can happen to make you feel better (e.g. positive experiences in a redeployed post), but in my experience little of your recovery and renewal will be related to having your grievance upheld, or even to "punishment" of the bullies.
HTH