Hi OP,
I'm really sorry to hear you're going through all this - I've a lot of experience with my own MH demons and how it can impact work. I really hope things improve.
IANAL.
A few thoughts:
There are potentially GDPR implications if your boss didn't give explicit consent for their personal data to be shared. I would have asked my partner to message him from my phone, and just say something like "hey, this is omnomnom's partner, I'm messaging on his behalf because he's unable...". However, as there's a clear and legitimate interest for having shared the number I imagine it would be viewed as justified by a most. Also as this was a personal number freely shared, I doubt the company's privacy policy could reasonably be applied, but would depend on the policy itself. I would be astonished if anyone would pursue any sort of litigation along these lines given the context, and even less chance that they'd succeed. But, some places are petty as hell.
Is it relevant that you have evidence that your password was changed and your work data was shared - this would depend massively on their policies regarding acceptable/personal use. Article 8 of the ECHR specifically protects a person's right to "respect for his private and family life, his home and his correspondence", so if they logged into your computer account and you have evidence that they ended up in your personal e-mails, they've probably crossed a line. However, logging on to the account to find work related data which, generally speaking will be owned by the company, is probably fine.
It's quite reasonable not to follow absence procedures if you're unable due to a health condition - ACAS would tear this apart.
Telling work that you were admitted to hospital via ambulance and probably wouldn't be in the following day, but then showing up, does send mixed signals. Even though it came from a good place, someone will say something like "can't possibly have been that unwell" (even though this is obviously untrue). I've found it's quite important to set expectations as stick to them as far as possible. Also, documentation of everything will be essential.
You really need ACAS' help here. My impression is that they're trying to manage you out.
All the best, OP.