Best approach for building a webshop with more than one URL to the same product (entry)
My two cents:


I wouldn't use different url's for the same product, that's considered bad seo. Instead use something like domain.com/product/slug. That way you can change product categories while keeping the links intact, and it'll be easier to get the url of entries.
If there is no specific reason for using structures, use native categories.
Instead of reverse-relations for seo-information, a simpler solution like Sprout SEO might be better.


tl;dr: keep it simple