You say in #1 "A few years ago . . . ".
I believe it is now far too late to take any action over this, even if your copy is pirated. You bought it in good faith from a seller you had reason to trust, just be grateful it works and hasn't infested your computer(s) with malware. I might be reluctant to deal with that particular supplier again - if they are still in business - but again it is always possible that they were scammed. It wouldn't be the first time that a reputable company has been fooled by a supplier themselves.
I think if this is a possible illegal disc, I think the seller was scammed too.
I have some more stuff if you're willing to help.
On the Internet, it talks about how legit discs are "stamped". I'm not sure what this "stamped" appearance looks like (or if you can even see it), but I have been comparing the disc with other games that I have and I notice that the legal discs have this wavy circle on the bottom of the disc. Maybe you can see it, maybe you can't. Not sure if this is the "stamped" look, but I don't think a burned disc could produce that specific appearance.
Also, if it is an illegal disc, would the person making this even bother to put the copyrights anywhere on the package? I've read that copyrights and company logos can be missing, but I haven't seen this. The copyrights and logos are present.