[1] This should be OK, but best to test.
[2] There should only be a placeholder in the mobile variant, not the desktop variant. This is my test site:
https://initiostar.co.uk/demo/banner-vid/ (different video using fixed not absolute, but absolute would invoke a scroll). The mobile code can be seen when you squeeze the browser; it uses a simple video tag which should be in a mobile variant placeholder. You can as you suggest just use an image, but the video should work too.
I'm sure we can get it to work.
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