It would be really interesting to see the same analysis for verified reviews only, and contrast it with the overall numbers and non-verified reviews. I would actually want to read that more than this (which was still interesting).
Do you mean reviews done by verified purchasers? I believe I have read that firms buying reviews get around that qualifier by giving the reviewer money/gift cards to purchase the item, then write a positive review. They could easily set it up so that the reviewer has to send back the sample item, too, to reduce the cost.
Exactly this. Amazon claims they're fixing it but I don't think that'll stop the problem. If anything it'll cause them to do reviews without posting a notice about it being sponsored or potentially biased.
Fiverr is full of people offering to do reviews on this basis, for example.