Hello there. It does seem like you've got a lot going on at the moment, both with work and your health, so it's probable that some of your health issues may be impacting how well you're coping with the ongoing work problems. That's certainly not being said to diminish in any way how you feel, just to make comment.
Moving on.
You don't mention how long you have been signed off for, so I'll take a flier and assume it's a month/4 weeks.
To be fair to your employer, it sounds like they are trying to support you. If you are off with work related stress, at least they aren't just leaving you feeling abandoned, as some people report they feel in similar circumstances. However, as with many things in life, timing is everything.
Bearing in mind my assumptions around your time off, in your shoes, I would consider doing the following:
I would decide to use my time off to try to learn as much as I could about the newly diagnosed diabetes. This is a marvellous site, and I learned massive amounts from here when I was diagnosed almost 2 years ago. Ask plenty of questions, and people will help. Getting a handle on your diabetes is likely to buoy your mood, aside from anything else. You may be feeling "off" due to your diabetes, whether you realise it or not. Some people also do have headaches with uncontrolled diabetes. I'm not saying that explains your other issue, simply making comment.
Secondly, provided you are not expecting your employer to make any modifications to your workplace, and also assuming your job doesn't involve using dangerous machinery, I wouldn't feel for a moment you have an obligation to tell them about your diabetes, and certain not right now, and not whilst you are signed off work for other reasons. Your time off may allow you to think this through also and help you to decide how you will handle the discretion/disclosure dilemma (again very common), with the benefit of some time and knowledge under your belt.
Thirdly, I would contact OH and tell them that you don't feel up to a telephone consultation at the moment, and would really prefer something face to face. I would want to have a little time to move myself away from the presumably acute phase of your stress and anxiety. It's much easier to be balanced and negotiate a decent outcome when you're not feeling burnt out and hyper emotional, as can be typical at the stage you're at.
I don't believe it would necessarily be helpful to attempt to put OH onto a "don't call me, I'll call you" basis, but maybe you could agree a catch-up call in a couple of weeks to see how you feel about a meeting then? It is probably important you don't take this too quickly, as sometimes over-enthusiasm and hyper-commitment to an employer can lead to stress becoming a bit like a revolving door. The sufferer just comes off the bottom and goes back to work, but quickly tires and starts to sink again.
I'm not encouraging you, or anyone else to push the envelope and stretch the time they have off work, but it might be worth an open and frank discussion with your GP and see if he/she can give you an indication of how long you might expect to be away from work.
I've only been off work once with s stress related issue; after a difficult bereavement and starting a new job I didn't enjoy on the day back after my compassionate leave. I eventually just hit a wall and needed time away. At the outset, I thought a week or maybe two weeks. In the end, once I began to realise how up-tight and emotionally drained I was, I was away from work almost 3 months. It did me the power of good. I didn't miss the person I had had lost any less, but it allowed me to get into a better place to move forward from a role I really didn't enjoy, working for someone I couldn't respect. Looking back, I realise how fortunate I was being internally head-hunted a couple of months later into a big, big, challenging job that I loved, through to the day I left.
So, just take it steady and don't rush into things. A period of distance is probably important, and I'm pretty positive your employer won't object - provided you keep in contact with them, and make it plain you want the best outcome, which you believe will be best achieved by a little bit of separation, in the first instance.
Good luck with it all. You must feel like your juggling too, too many balls at the moment.