Unbreakable has a chance to increase Willpower by 2 to 5 points, 85% of the time -- which is pretty much guaranteed.
The unbreakable trait can't go away, slowly or otherwise, because willpower needs to be below 25, which doesn't happen because math and not luck.
I'm just trying to help. I just looked it up, it is 2-4 80 percent of the time.
<<if _npc.traitun gte 75 and _lossroll100 lte 80 and _npc.willpower lte 59>>
<<set _npc.willpower+=random(2,4)>>
<</if>>
<<if _npc.traitun gte 75 and _npc.willpower lte 24>>
<<set _npc.traitun-=1>>
<</if>>
Nonetheless, let's focus solutions.
If you still can't lower her willpower, you can just reload to when you first purchased her and right before talking to her.
Otherwise, leaving her in upgraded bond. cell have a chance to lower her an extra willpower.
I'm out of ideas, maybe someone else can help.
Edit: Oh ya I just remembered, spanking works really well, especially if her corruption is high. Orgasming while being spanked is guaranteed -1 willpower, but spanking in general have a small chance. It requires a bit of stamina from both parties, but she has good stamina, so, try that maybe.
Edit2: You need high flagl. skill to do that I think. Just read some books in bond.
Edit3: Don't put her in bond. room, just ask her directly for spanking and if you your success is 80 or higher, you are good to go. If it is lower, just need more practice+books+courses. If you fail spanking, there is a chance she gets willpower. I would say stick to the 80+ rule.