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It's possible that the canonical timing was just a coincidence and something deeper is going on, but I look at it this way - if it's easy to fix, fix it, and then you'll know for sure. It can be really tough to separate technical indexation problems from penalties.

Losing PR does not necessarily mean that you are being penalized by Google for keyword stuffing. Have you noticed a change in rankings or traffic? It sounds like the cause is between an inflexible CMS and a client who has their own ideas. Would changing the product title really hurt their traffic in e-commerce engines? Are there any products with traffic that the client would be willing to test? Can you gather any data (e.g. traffic/competitive data for a more ideal product name) to show what they're missing out on? Some projects include more diplomacy, convincing and teaching than technical knowledge or link building. This might be one of those. Without seeing the pages in question, I can only offer my best guess.