[Unhinged Manchild]

Interesting scenario from a recent game of AotP:

This was in a 1v1 game between Jace Beleren, Mindmage (the one with Mind Stealer, not the crappy newer Jace) and Nahiri, Fury in Stone. Nahiri summons Flamewing Phoenixes onto the field and then Nahiri is enchanted with white enchantment Not Forgotten (I am pasting enchantment text here because this is primarily where my question lies - Enchant Planeswalker - Whenever a spell or figure an opponent controls destroys a creature you control that's adjacent to enchanted Planeswalker, put a +1 power [attack] counter on enchanted Planeswalker.) In his following turn, Jace succeeds with the Mind Stealer D20 roll and the player moves Nahiri into adjacency/engagement with one of the flamewing phoenixes and destroys it in combat. Does Nahiri's Not Forgotten enchantment trigger here and give her a +1 attack token?

My interpretation: It does trigger, since Jace's mind stealer ability says "...16 or higher, you control that Planeswalker..." and Nahiri, now a "figure an opponent controls" which destroyed a creature (creature in control of the original white/red player) which is adjacent to the enchanted Planeswalker, meets the criteria to earn a +1 attack counter.

Thoughts on my interpretation, should this be ruled differently?

Side note: Eldrazi Ruiner + Swift Justice + Ghostly Possession is devastating against the high defense blue squads, and has a high chance to heal the Ruiner for a truckload of health given the target blue squaddie has no wounds before the Eldrazi attack.