How and why are Google signing in to the outlook.com account

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.

It is possible to set up an alias for your email address in the country code top level domain. The option is available somewhere in the mail settings. Here are some steps (note that this is using Outlook online):

1. Log in to your Hotmail/Windows Live account.
2. Click on the gear icon on the top right hand corner and select "More mail settings"
3. In the Options page, click on "Your email accounts" under the "Managing your account" header.
4. In the new page, you will see the "Create an Outlook alias" heading, as shown below:
5. Click on that button and you would be able to walk through the creation of an alias in your preferred domain (depending on availability of the email address).

Hope this helps!