In cases like this it is often easier to just provide the link to the page on production for us/someone to debug more quickly.
They do not require pre registration.
No
Yes
(This section edited for clarity): Not for the parent domain itself (DNS validation)
Parents are just the domain (no https:// no path) which is valid when using window.location.hostname
In this example Twitch Embed example I use embed everything and don’t need to declare a parent expilcitly as the JS lib does it for me.
I suspect the issue here might be nested iframes but I can’t debug that without knowing/seeing the webpage with the fault. (and in that scenario each frame on the way down needs to be listed in the parent(s) see this google sites example: twitch_misc/player/googlesites at main · BarryCarlyon/twitch_misc · GitHub // Twitch Embed Test/Example code for parent issues )