What is the likelihood that an xPub child address is not owned by the same xPub address holder? Kind of an odd question, and I'm pretty sure the answer is, "extremely unlikely," but wanted to see what the community thought.

Secondary question: How sure can one be that the addresses walletgenerator.net and similar paper wallet generators generate are not all related to an already-in-use xPub?
If you assume that an address is generated from a xpub/xpriv keypair, then yes, it would be safe to assume that the owner of that address also owns the xpub/xpriv keypair that that address was derived from. The probability that someone was able to generate the same address randomly or that someone was able to generate another keypair which also has that address as a child is so low that it is effectively 0.