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Hello there, from what I've read, so long as you write unique, interesting natural content, you don't need to worry about keyword stuffing. If content flows naturally and gives a lot of value to users, it should be fine. Unnatural keyword usage looks like this: "Bob's cheap Seattle plumber is the best cheap Seattle plumber for all your plumbing needs. Contact a cheap Seattle plumber before it's too late." If you are worried, maybe try spreading the keywords around, using synonyms or adding more valuable content to thin out the keyword mentions. Hope this helps.

Video are good, and have always been good for the SERPS. It is always good to have keyword relevant content around your video embed, especially as the H1 title of your video. Google doesn't hit you for adverts above the fold unless it is a lot. Look at it from a user perspective. When you open your page are you blasted with adverts. A few nicely placed are okay, just don't have all adverts and very little content above the fold.

Again, it's really hard to hypothesize without knowing the specific site because there could be a ton of other things at play you may just not be seeing. Videos are content so if they have a lot of videos, then they don't have 0 content. Especially if they have a transcript or CC file attached to it that can be crawled. And, if they have a video sitemap and how their hosting is set up all factor in to some degree. Even with the changes to Google's algo, links are still powerful, especially links from .gov or .edu - just a few of those can mean more than tons of links from regular .com sites.