Is it a small or large company? How long have you been there?
I'd be inclined to think you have a manager whose panicking a bit and doesn't know what to do next, so they're asking everything they can to make sure they don't miss anything.
It's a little odd that you're back at work full time, no adjustments, before the report has been done. It's normally part of the return to work process.
You absolutely, 100% have the complete right to refuse the GP access and insist on the Occ. Health referral if they want answers. You have the right to refuse an Occ Health report. You have the right to have the referral, see the report 1st and then grant permission for data to be revealed.
You also have the right to know who at your employer is going to see the report, and - this is important - this needs to be as small a pool as possible and only those with clear need. I would expect the HR person managing your file to receive the report and then pass on recommendations to your line managers etc. I would not expect the whole HR dept (if you have one) and then everyone in your line of management to have full access to the data.
The report should concentrate on your condition now, and your fitness to work. The question 'outside of work' may matter, because if in, say, six weeks, you make a serious mistake, or are underperforming, knowing that you are still suffering outside of work will have an impact on what, if any, level of disciplinary or management to bring to that.
That doesn't mean they need to know everything that happened in detail. The answers to questions are likely to be more 'PTB suffered significant and serious repeat trauma both from the event and the consequences of this, and is now experiencing x, y, z as a result, ' rather than giving explicit detail.
They cannot sack you for refusing, but refusal does mean your lose the opportunity to make adjustments which might be to your benefit and they'd be able to 'ignore' it in the future instead of having to accomodate any ongoing issues.