Q: Can I only import reviews and nothing else?
A: Yes, you can do that technically.
You need to specify attributes wp_amazon_asin and wp_amazon_locale for each product which you want to download reviews.
Navigate to Product Information - > Amazon Import Products tab. Here you can find "Amazon Local (option)", "Amazon Standard Item Number", "Sync with Amazon (option)" fields.
Amazon Local might be the same for each product. It is like 'us', 'uk' or other locales. Amazon Standard Item Number - is an ASIN for each product. ASIN is required to match a product on Amazon with a product on your Magento site. Sync with Amazon should be set to 1, it will place product to the sync pool.
You need also to remove all fields from the sync list, so that the syncing process does not overwrite your product's data.
After enabled synchronization, enabled loading customer reviews in the extension settings - products after sync should get reviews.
Navigate to Configuration - > Amazon Import -> Synchronization settings tab in order to set up "Updated product attributes".
Note. Reviews are not loaded instantly, since they are parsed from the site. Reviews would be loaded during synchronization.
Video tutorial on how to import only Amazon reviews
Amazon reviews downloader and parser extension will allow you easier and more effective grab reviews from Amazon sales and post it on your eCommerce site.