Why would the council make it so wide if not to fit two cars?
Probably so that large vehicles have enough room to safely negotiate the junction and join a faster moving dual carriageway.
I understand that at this sort of junction it is common courtesy for drivers crossing the dual carriageway (straight on, or turning right), to position themselves to the right, allowing other drivers to pass and turn left.
No-one is debating that, in fact most people will have been in a situation where they've done similar to your wife and moved past on the inside.
What people are saying is:
a) the vehicle in question was a large vehicle (larger than a car), so more caution should have been applied. A more defensive driving style would have avoided this accident.
b) even if 'everyone does it', doesn't mean you can expect to be blameless.
No-one here can give you a firm answer on how this will go, all we can give is our opinions. It may end up that the van is found at fault and you can come back here and tell us all how we got it wrong. But all we have to go on is your explanation of the events, and from what you've told us, most people seem to be in agreement that at best your wife was at least partially at fault.