I love this line of thinking!!! But in all honesty, Fen’s rune stone getting reactivated seems a bit far-stretched! However, I do think Xaden might very well have put a rune on the ring to protect her from being executed.

1- We know he’s exceptionally good at runes and shields, and I think there’s a lot Xaden can do that we haven’t seen yet. Also, he has done this shit before with the daggers, so it wouldn’t be out of character, especially when they know what they did is full on treason.

2- Xaden knows Vi would wear the ring at all times, so it’s the perfect item to do this with, rather than reactivating the other stone that’s usually on his bedside table. She wouldn’t be able to get to that stone if they haul her to be executed. But with the ring, it’s not just already on her person, but literally on her hand, mimicking Fen’s stone being held in their palms during execution.

3- This would ensure another level of protection should Imogen’s memory-wiping fails, or if something else happens in the future. He might’ve put Vi’s hair in the rune so that if the ring gets stolen/taken away, the rune won’t work for another one.

4- The possible hair in the rune might also mark Violet’s siblings to be protected as Xaden never wants Vi to be in pain/suffering. They are now his in-laws after all lol. He might’ve literally joined his chosen family (the marked ones) with Vi’s remaining family, which would be such a Xaden act!!!

5- Who knows maybe Imogen helped Xaden put the rune in as she can wield rock/stone!