I think the simplest thing would be for the 15 owners to decide to continue with the current practice and turn a blind eye to the bylaw if all are of the opinion that nobody would benefit from enforcing it.
If the parking spaces on common property are not those designated as visitor parking (so can’t be reallocated to another purpose) and were built as parking spaces from the start, then clearly their purpose was for parking by residents. If they were unallocated, then it seems to me that your current practice is consistent with their common property purpose. It sounds like all lot owners get a chance to use them since people come and go; everyone has a chance they will find a vacant space on returning home. I would say you only have a by-law problem if one lot owner uses a space all the time (perhaps by leaving a car than never moves) preventing other owners from deriving ‘use and enjoyment’ from that common property.