It's pretty unlikely that a tiny "slice" that you need a loupe to see on the outer o-ring will ever cause you an issue. That one is mostly cosmetic.
There are three other o-rings inside the tube that do the sealing, plus one inside the crown, and that visible one is a third seal (hence Trip-lock)
Any decent watchmaker who works with Rolex can likely change it for you in a few minutes and so your AD probably didn't have one and needed to send it back to Rolex
No, you cannot stretch the o-ring over the top of the crown. The crown and stem need to be removed to do the job.
