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I don't think its illegal, like you are describing it, but you can get trouble with your aff network. Most affiliate networks don't want their members to spam and its against their TOS, you could lose all your income because of this. Especially when your are sending millions of spam messages...

Check if the site is linked to social media profiles, this could be a chance to contact the owner. If not, only whois could help you.

Try to find them on other searchengines. Also contact google, tell them that you have bought the domains, but you are wondering why they are not indexed, maybe they can help you.

It depeneds on the niche and the product. With a "brandname" you could review and promote different products form the same niche. When you take a keyword related domain name you are more or less bound the the keyword for the content of your site.