You are right on your first point. Renting out designated visitor spaces would require council approval as it would (probably) be in breach of your Development Approval in terms of the number of required visitor spaces and a change of use as well.
In the unlikely event that council approved this, the money would go into the Admin fund. Any excess amounts could theoretically be disbursed to owners on the basis of unit entitlements but that would require a unanimous vote of owners. Otherwise it would be used to offset the running costs of the building.
And just as a general observation, unless you have a huge number of unused visitor parking spaces, this is a bad idea.