I’m a regulatory lawyer. Sorry you’re going through this OP.
It sounds like the mistake involved giving inadequate advice, i.e. failing to advise of all potential options and recommending the most appropriate/commercially sound. That strikes me as less serious than giving advice that was fundamentally wrong (in law), or missing a statutory deadline etc.
Whilst there could be potential professional negligence involved, as a PP said that’s ultimately for the firm’s PI insurance to deal with - but obviously doesn’t stop the firm instigating individual disciplinary proceedings.
Re supervision, there definitely appears to be shared culpability here - but it sounds like you are reasonably experienced, and being realistic it’s not possible or practicable for supervisors / partners to check and review every piece of work line by line. That said, it does sound like the supervisor missed something pretty crucial in this case!
From my experience in investigating professional fitness to practise matters (and I appreciate this is an internal rather than SRA investigation), I can tell you that what tends to go in people’s favour is transparency/candour and insight. Owning up to your mistake, reflecting on how you’d approach it differently in future etc is key. However, they do seem to be taking a hardline approach which I agree doesn’t sound too promising.
Your life and career aren’t ruined either way though OP, though it might feel like it now- you’ll move on from this one way or another. Good luck!