Youtube – How do channel views work on YouTube on the about page

This is their background image. you can find it here: 2946 pixels wide by 1490 pixes high.

How you get the link at the top to work: Go into Channel Settings and upload the background image that you want.

Youtube partners are allowed to have a custom header/link in their channels. Here's a video that shows you how to add a banner/link to the top of your youtube partner channel. It's kind of a goofy video, but it's helpful

No, unless the owner wants to do it there is no way that you could claim another username. just find another name, for example mybusinessofficial or anything else.

It may be helpful to read the YouTube's username trademark policy.

Problem is, they're probably not infringing your trademark, as you may know, just using a name doesn't infringe a trademark, what is important is "how" they use it. that said, if they even try to upload any content, similar to yours, which imply that their channel is connected with your brand, then you may use YouTube Trademark Complaint Form to take down their content, their account or assign it to yourself.