To answer your direct question, yes, misleading signage is a valid reason to challenge, comes under ground of contravention did not occur.
I would however suggest that you spend the time to post up all documents.
People that focus on one ground that they think is strong often come unstuck.
If we are to discuss or advise, we need all the information.
An example for you which may or may not be the case.....
You parked right next door to the one sign that had been changed.
A motorist has a duty, each time they park, to check restrictions.
Given the above, an adjudicator could easily decide that the signage did not mislead.