As far as I can see your reasoning then, if Prince Charles wears wings and you're OK with that due to his appointment, having your cake and eating it comes to mind as the distinction seems a little moveable here.
As far as many people are concerned, the fact that some people have completed a Pre-Para or P Coy course doesn't matter. If the regs state that someone is qualified to wear the badge then it's the regs that are wrong, rather than the individual, surely.
I served underneath more than one cap badge and alongside some "old school" type 1970's ex-Paras in one unit. As far as they were concerned, anyone wearing RAF Parachute badges were not fit to do so, regardless of how they had been earned or awarded. Towards the end of my time, I heard some saying that anyone who had been awarded wings after balloon jumping had been withdrawn had it easy.
It's all relative at the end of the day. I have no problem with anyone wearing any trade or qualification badges, no matter what they are or how they got them, as long as it was done by the appropriate book. Equally, I have no problem with either Prince Charles or Prince Andrew wearing their wings.