Let’s see if I can help out.
Let me start by saying that the testing whitelist for an extension has nothing whatsoever to do with Twitch user account settings. Nothing. Zip. Nada.
The most common problem that is encountered here is that in the dev site, when you edit the testing white list, you need to first take your extension out of “hosted test”, edit the testing whitelist, and then move it back into “hosted test”. Whitelist edits are applied to your extension when a state transition occurs.
Please let me know whether this makes sense!