"If Perth had nominated John Mooney as a next star he'd have to pass on each club that had a next star spot"
No, that simply isn't how it works.
Whether we agree with the concept, the idea behind "Next Star" is to promote the League. It's not just an excuse to get another player.
ie: There must be some basis for considering the player a future NBA star.
The criteria was that they be eligible for the NEXT NBA draft. It has never been expressly stated, but I think is reasonable to suspect that the league applies some quality control. eg they make sure the player is actually a reasonable prospect, and clubs can't simply sign any kid they like.
The criteria was expanded to include "Stashed" NBA draftees. Whilst it stunk that Sydney was again the First team to get an advantage, it is within the aims of the program. ie: They are a future NBA star.
Players who went in the draft but were not drafted are not eligible, for the obvious reason. (The League might in future allow a genuine 19yr old prospect who nominated a year too early)???
So no, Mooney was not eligible.
Neither was, say, Wani Swaka, because even though he might have been technically eligible for the Draft, his chances were zip. The League knocked back another young Aussie that had a couple of clubs interested.
Giddey was highly rated and considered a genuine chance, hence why he qualifies.