Yes, indeed! However, according to me there's no need to insert h3's in your product/category pages

It's not a big help, but I figure if you can differentiate the important text from the useless text, why not do it?

Yes, of course. It's not a big point, I can't see a reason why it would hurt your rankings.

I saw on your website that you have removed the pagenumbers (Px)of the html filenames of let's say, the category index pages (?). Could you tell me how you've done that??