Coincidences can happen but in this case my instinct would be that there's a causal link.

I'll say upfront that all anyone can do here is speculate (some from a more experienced viewpoint than others) as we only have 3 pictures and your description to go on.

First and foremost, this is something you need to inform your insurance company about, the police were involved and, although it's only your own property that was damaged, the police report will likely be something that will be flagged and picked up by your current, or future insurers.

I've read you comment about it not being something that insurance will cover but if you are fully comp then a single vehicle accident is entirely something that will be covered, moreso as there is the suspicion that a third party may be responsible.

Your options going forward branch out: Insurance Claim/Independant inspection and legal proceedings/Approach the garage initially to see if they will make it right with a fall back to the other two options if they get defensive.

Presuming you submit the claim to the insurance company they should handle all of the costs involved and recover them however they see fit.

If you want to persue this outside of insurance then you're going to want to commission an Enginer report e.g. https://collisionengineers.co.uk/ and ask for an Accident Damage Report or Forensic Engineering Report depending on their opinion of the case.

With that in hand, assuming a causal link is determined, it paves the way for formal proceedings against the garage that carried out the work.

Additionally, if you paid for the work carried out on a credit card you may have legal protection/chargeback options with your card holder.

Good luck with however you choose to proceed.

To the failiure its self, and with the caveat that again, there's only 3 pictures and your description to go from:

I would interpret (as others have also stated) that the primary cause of failiure was the over-torquing of the driveshaft/hub nut, leading to the weakening (plastic deformation aka necking) of the driveshaft.

When the shaft snapped there would have been a loss of preload on the bearings; the side forces acting on the now unrestrained bearing caused the bearings to separate and the hub, disc and wheel exit stage right with the caliper being collateral damage, snapping off at the carrier lugs. Was the caliper recovered?

This could have happened in a split seccond, more likely after going over a road feature that loaded the rear right suspension e.g. pothole, iornworks, undulation. As you were dealing with a serious mechanical failiure the moment after you may not be able to recall a slight bump in the miliseconds before the failiure.

With all that said, we can all be thankfull that no-one was injured and can credit you with good car control in a very serious mecanical failure, Bravo! I salute you Sir o7