When going through a list of several urls, it's hard to keep track which videos have already been queued. Make permissions for those websites optional.
If the videos later get un-merged, either by user unlinking the uploader's accounts, or by a collection website listing the other video only, the listed video should show up as already watched.
It may not be obvious what some of the non-video links point to. Chrome doesn't display where it points to if you hover your mouse over them. Could show where via a tooltip on top of the link, or at the bottom left/right of the popup.