You are the guru for everything I love and am starting to love (like Tiffany’s and now Chanel shoes).Chanel will fix defects. I had a tiny CC gold logo fall off, and Chanel replaced it for me.
My friend has the Chanel CC crystal earrings, and one of the crystals fell off. Chanel repaired her earrings. Both of these issues occurred within a year of purchase, so the replacement piece was still available. I would think if a crystal falls out 25 years from now, they may not have a replacement. As FYI, for Chanel repairs of bags/shoes, there is a company called Santana Leather that does amazing repairs and gets replacement parts. I think they actually have a repair & drop off inside Bergdorf Goodman which is how good they are. Here is their info for future reference: https://www.santanaleathercare.com/
I don't think the top coat of nail polish will do anything in terms of adding durability. It's just just nail polish and will wear off, so I personally would not do that. If you are really really really worried, you can send it off to add extra "crazy-glue" to the base of the stones as that is what really holds in the stones. For me, I would just leave it and if it ever happens, I am certain you can get it resolved.
Congratulations on your new sandals!
I always appreciate your thoughts and suggestions.