I bought a theme stating it has rich snippets and it didn't ... I used @nemo guide to implement product schema and may be magic to implement it in the default review module :) I don't know how, but I did it and it passes rich snippets testing tool. Just the price snippet has issue with the currency comma-dot separator, but at the moment google doesn't display price tag in my language results so I don't bother about it. It is 1.5 store, I don't know if 1.6 has snippets by default...
About keyword density and semantic entities you are completely right but in most cases merchants need to use professional copywrite and SEO services I believe.
Looking forward your review about AMP ;)