Youtube – Reclaiming Inactive YouTube Username

You can unlink your old Google account from your YouTube account. In YouTube, under Account Settings->Manage Account, there should be an option "Unlink YouTube and Google Accounts." Once you unlink them, you should be able to link your new account with your YouTube account and multiple sign in will play nice.

YouTube accounts created after YouTube joined Google are considered part of your overall Google account. There is no real way of separating them (you couldn't separate your Gmail account from the rest of your account, per se). I have the option to unlink because I created my account before that, and Google sees my YouTube account as an account that just happens to be associated with a Google account.

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.