1) My feeling is it is quite reasonable that if you allow one person to park in front of their garage you should allow all residents to do the same (whether it’s a single garage or double).
2) As for visitor parking it has been ruled in QLD that a visitor is someone whose primary purpose is to visit a resident and not have any secondary reason. So if your visitor is staying and going to work or study each day their sole purpose is not as a visitor and so is ineligible to use the visitor parking. The same reasoning should apply Oz wide.
long term you could also pass a bylaw defining limits on the time a visitor can park in a visitor parking bay. Max of 1 day per week for 8 hours or max of 4 hours per day.