Or the comment sections could just be merged together in the client view. Each thread of comments would belong to one (and only one) instance, so it shouldn’t be difficult to merge those lists together when presenting the aggregate view to the user.

If you want to share a video, unless it is your own, original content, you should be linking to the original source, not uploading the video itself to Lemmy. Uploading content that for which you do not have rights to a platform is freebooting, and is theft. Linking to the original source lets you share the content with others while still providing the views and revenue to the actual creator, ensuring that they will be able to continue making the content that you have evidently found value in.

tl;dr - link to the original source, don’t upload someone else’s video to the platform.