Hi I am very sorry to hear of your difficulties, but Nursing can be quite brutal and they will withdraw you.
Have you talked to the SU advisor or the NMC? NMC have a helpline, but am not convinced they can help except to say use the appeal system at the university. Do you have any good friends who are good at reading rules and will support you or any law graduates?
Its a case of someone will need to read the rules and then check on what grounds you can appeal on or see if there are any alternative options.
University rules are the dullest of the dull to read through and depending on the uni they can be extensive or obscure or both.
I make the following points which arent an answer or even the start of one, but are things I would note.
1., Your research on 320v 360 credits makes sense and have no doubt its correct.
2. Your research on it needing to be an honours degree or 360 credits also sounds correct.
3. You state you are only allowed one extenuating circumstance? Presumably you have checked that ion the rules.
4. I would check with the OU to see if they can and will take a transfer of credits. They might let you transfer 320 credits , leaving you to make up 40.
http://www.open.ac.uk/study/credit-transfer/5. You could consider appealing but ask to retake the year, sure they wouldnt make you do the placement again. That would only be at the end of the appeal.
6. Someone could find a reasonable grounds for appealing they consider acceptable and they let you resubmit for a third time. Have to say without reading the rules it would be a long shot and you would need medical evidence to back you up. Everyone has had to deal with covid 19 and the way unis take it is if you submit work, then you are prepared for it to be marked. Did you get a tutor or anyone who could give you some pointers on your dissertation? Because you were trying for the second time it was imperative you knew it was going to pass before submission, so a second opinion is always helpful. Considering what was resting on it then I wouldnt have left it to chance and sought an extension. Have you been working as a nurse in hospital and doing extra cv 19 hours? If so how much? That might be grounds. Did you make friends at hospital supervisor , doctors etc, maybe they could write you testimonials and attest to what was happening on the wards and your competence?
In any event it will be hard. Someone is going to have to read the rules, ideally a lawyer.
7. Yes you are going to have to tell your new employers what has happened as soon as reasonably possible. I dont think getting a job if you had qualified is the issue, although its disappointing.
4. wlould be the quickest and my favoured option if they can take you. Check with the NMC that would be acceptable either as a transfer i.e 320 credits to OU and they add 40 to get na OU degree. It may be if that works then you would have to ask your current uni to let you transfer credits rather than them crystalise into an ordinary. Alternatively whether they would register you if you got the extra 40 credits from somewhere. They will know what they have registered in the past. I would rather have the 320 credits rather than an ord degree as the latter is of little use.
8. If you managed to get this resolved in 120 days that would be very quick imo.
9. As far as I can see youd be looking at another year if they allowed you a way out because you cant get past the fact the dissertation wasnt good enough.
10 If I was doing another dissertation then I would want more time to make sure I started from scratch and knew what I was doing. In this situation if you got another chance afeter a lot of struggle you simply couldnt fail again which means patience and more application over time, considering whats at stake.
In any event have a word with OU and NMC on the points I raised in 4 and 7.
Thats all I can think of off the top if my head. I understand its a time of great stress and mixed feelings but do try and avoid beating yourself up or self flagellating over it. Making yourself feel worse wont help, you need to conserve your energy and apply it in things that will help or offer hope, even if remote. It can be exhausting. See how you get on. Good luck.