Would the same "proportion of blame" be applied in the case of a cyclist having a bike capable of assistance above 15.5mph? The assumption seems to be that, if you have a bike with illegal capabilities, you'in the deep & smelly no matter you weren't using those capabilities to the full.
I assume the same process applied to windows of illegal transparency, so riding an illegal bike is not necessarily going to lead to liablity in the event of an accident?
First of all, in manslaughter there is no "proportion of blame". The only time that becomes a legal issue is in the civil courts when damages are being considered. In criminal law there is (at least in England, Wales and Northern Ireland) only guilt or innocence (Scotland has the rather interesting "Not Proven" verdict, too).
In this case the charge was manslaughter. The jury felt that, given the evidence they'd heard, the man wasn't guilty.
Should the evidence that was presented have just been slightly less than that required to prove guilt, beyond reasonable doubt, then I would have expected the jury to consider it for more than just an hour. I think I'd also expect to see a civil claim being made against the driver, as the burden of proof for a civil case is lower, just "on the balance of probability", rather than "beyond reasonable doubt". If there is no civil case, then I think it would be safe to conclude that the evidence against the driver might be very slim indeed, and strongly outweighed by other evidence.
With regard to the tinted windows, then these may contravene the Road Traffic Acts if they are darker than whatever is allowed, I'm not sure. Not a serious offence though, I'd have thought.
The case for an illegal ebike would be a breach of statute law but is potentially much more serious. In essence the charge could be riding an unregistered, uninsured motor vehicle, that has no form of valid Type Approval, failure to display a registration number, failure to display a valid VED disc, perhaps failure to wear an approved crash helmet and maybe even riding a motor vehicle without the appropriate licence.
As always, it depends which laws you break as to how serious the consequences might be. Having windows a bit darker than allowed doesn't change the category of the car in question, doesn't nullify its insurance etc, etc.