1. Yes a video background does effect your SEO see
    https://support.google.com/adsense/answer/6196932?hl=en and
    http://searchengineland.com/mobilegeddon-beginning-not-ending-220512
    
    The slower your site loads the farther Google pushes you down the
    SERPs. The biggest concern for Google is mobile devices. 
    
    So what should you do instead of adding a video background? Research
    Sales Copywriting, customers care about your website solving their
    problem more than anything else. It's hard for a video that's a
    background to solve any problem unless that customer is looking for
    flash.
 2. Maybe, but the main goal of your site should be to create something users love. Your website could look terrible and very plain and outdated however if users find it valuable you'll get backlinks and organic traffic. See http://craigslist.org for a shining example.