[Quoth]

1) Maybe. If the video was in the ebook, you'd not need an iFrame.
2) The video is a link to YouTube. You do that on a Website. The video needs to be in the ebook, (which makes it interactive media and not really an ebook)
3) You need a video file, not a link, so, no href except to a file actually in the ebook.

You have not embedded a video in an ebook. You have used epub3 as if it's a web-page. You have only put a link to a video. You need the actual video file.

An ebook might have a link to a publisher's web site or something. But the actual ebook, entire, has to work on its own without the internet, otherwise you might as well offer a link to a website with the iframe and whatever else is in the ebook.

Most distributors of ebooks and ereaders and ereader apps won't do video.

You can do loads of stuff with epub3, but not all of it makes sense as an ebook.

Putting a link to a video isn't embedding a video. It's embedding a link:
viz:


The above is your link embedded. The video isn't embedded on MobileRead but is still on YouTube.