I feel like I'm going into the lions den and I'm really scared.
Good luck, OP. I'm not surprised you're scared.
Some advice - it might be useful (or it might not
), but something to think about anyway - even if you disagree, sometimes working why, can help work out what you want to do.
I was in a situation of facing an investigation (not for abuse, I hasten to add) - it was a malicious complaint made against me which was eventually found not to be worth taking forward past an informal discussion. However, I could have lost my job.
I spoke to our employee assistance people for advice, and one thing really stuck with me. The advisor said to me that I didn't have to say much. I should let them do the talking, in order for them to substantiate the allegations.
Now, you're in a very different position, as you are making a complaint (and not a trumped up one). But I think that the 'not saying too much' is still really good advice.
Maybe you could simply ask a question. Something like:
"I am medically signed off for stress and its negative effects on my physical & mental health. What can you do to ensure I have a workplace to return to which will not make me ill again?"
or
"How will you ensure that I am safe in my workplace when I return?"
And then let them answer. Keep silent for longer than might feel comfortable.
It is not down to you to solve their problem.