It has some impact bust not very serious, I would say converging to zero. I think writing a good description that answers the serchers's querry and generates clickthroughs is more important than stuffing your kw in the first few words.

If I get it right sites are based on the same topic, so interlinking them carries both user and search engine value (maybe a little bit less for engines than for users as google maybe knows that you own both sites so it gives less value to the links). When you do your linking just keep in mind what pages would you like to rank with and hw pagerank flows so do your job based on that http://perthseocompany.com.au/seo/tutorials/a-simple-explanation-of-pagerank (thanks ALan). Good luck!

Repeating just a snippet of infromation is not considered to be duplicate content. So if you have a few lines of text appearing repeteadly somewhere on your site, or multiple occurances of a video taht is not a duplicate content as long as the page contains enough individual information.

The other thing is that engines can't really find out video content they just make conclusions based on the descriptions you give them. Vary descriptions and titles.

It is legit as carries user benefit. I'm pretty sure that is forwards link benfits as well.

I think this is not real SEO itself, but rather cross-marketing or brandring issue, however owner of all brands is the same I suppose.