Wouldn't it be possible to simply get an extra small shoulder panel, or are those not available?
Your wife's experience is interesting to me since it just proves that the same equipment can fit people so differently. I also dive a Zena, but often don't even bother to pull the shoulder straps tight since I find that once the 4 body straps are tightening down, there's no movement or shifting of my BC and tank. I also found that I needed the shoulder straps tightened even less when using Steel 95 tanks vs. AL80s.
I'm absolutely NOT discounting her experiences at all, just am fascinated how different experiences can be between people. I hope she finds the perfect solution.