Problem with publicly commenting is that everyone and anyone will jump onto it, whether it's their business or not. This is exactly what happened when his son commented on fb and people started saying theyd batter/kill him etc.
Clearly this needs sorting between Carl and the OP in order to get any decent resolve.
The car in question should either return to Carl for the work completing (which I doubt OP wants to do, understandably having it taken so long to this stage), or get it priced up at another company for the work rectifying at Carls cost.
As it stands everything needs putting into a timeline, with proof (which hasnt really been provided throughout the whole ordeal) so that it can be resolved. Whether that be screenshots or invoices etc. Carl's not denying the issue has arisen through lack of time but I wouldn't doubt had it been completed the workmanship would have been decent pushing good figures.
Ultimately it's a huge shame its come down to this, the way OP has originally gone about the whole thing is wrong (can appreciate the red mist and being pissed) with
@steviekyle13793 3 s rendition of events making things much clearer.
I think the assault has also been a mistake and will be leaving a bitter taste in both parties mouths.
The car needs returning to the standard it started at, with additional costs recouped. Cant really hope for much better tbh.