On the subject of blue badge parking in Scotland, the law is pretty clear:
You can use your Blue Badge to park in certain restricted areas for free, including:
at on-street parking meters.
in pay and display bays.
in disabled parking spaces.
on single and double yellow lines if it’s safe to so and there are no loading restrictions.
This was copy/pasted from the Parking and Blue Badges page of MyGov Scotland, so whoever told you you couldn’t park on single yellow lines was wrong (except if it was not safe to so so/under load restriction etc as above).
In life, it's not who you know that's important, it's how your wife found out.