Will's always had his own staff since adulthood and I remember very well that press corps were crapping their knickers being called into BP at the time, last minute (!!), no brief on what it was about. I never forgot that and that was a bit of an indicator that the rumour of the Windsors being basically blind sided was true.
All it needed was Will's staff to call the press corps (royal rota) and BP to be notified to let them in.
Also the Windsors were told before the press, about 2 hours before the conference and the press corps were called in last minute and had to drop everything the moment they were told to get to BP, that was well documented by the journos at the time.
They have now confirmed the rumour and all this themselves through the docu special, which their hired wedding staff (baker, embroiderer, security etc), friends and family (Uncle Fester Gary G) took part in, so you can bet it had final approval via KP.
Will being "too correct", please... The stories are he's anything but. He's often went his own way and is said to be a bad character (acc to insiders, journos, and the public tiffs between him and Charles (and Harry)).
The ring would've been in the vault with the other stuff the brothers had at the time jointly access to, they lived together (at Clarence H at the time and then W&K&H lived together for a while at CH with C&C) and shared most things, if not all.