What's the purpose of the URL if there's not even any sorting or anything unique going on? If's a sorted URL (say by "size" smallest-largest for /little leage/ URL) it might be actually useful to develop some unique category content to let the page rank separately.

If the content is totally unique, I don't think you could really go wrong redirecting. To be safe, I'd probably rely on analytics to answer the question "what impact will redirection have?" For instance, is there a difference in conversion rate between the URLs. If you see a conversion bump from a more specific URL, you might want to sleuth out what's causing it.

Yes, providing that the /category1/ and /category2/ heirarchy doesn't help the user experience (e.g. product segmentation based on say, color and brand, which would be useful for users to drill down to).

I like 301s better because they are permanent, non-ambiguous, respected by all engines, and chiefly because they eliminate the possibilty of inlink dillution because the redirected URLs are never seeen.

Miles,

On your last question, I'm wondering if those two canonical tags are necessary? Are the /sub/ versions of those pages necessary for user experience? If not, I'd add a canonical element to the .com version, then redirect the /sub/product.html to /product.html. That would help you avoid splitting link authority.