I believe most e-ink devices, which are what the majority of digital consumers read on, would not support a video cover. As you mentioned, you can use an image for the cover and tag it in the manifest. The video can be added as one of the frontmatter sections of the book in a separate xhtml file, using the proper accessibility guidelines for videos in EPUBs. (Refer to: https://kb.daisy.org/publishing/docs/html/video.html)
I don’t believe there would be an issue using epub:type=”cover” for this section since this is what the section is intended to be. You can also mention in the accessibility summary that the book includes a video cover, and it can also be added as an item in the landmarks and EPUB TOC for readers to navigate to.