Yes, you can use multiple Google accounts.
no
It's free with a Google account. Google accounts are also free.
Looking at the mainframe, I do believe over 9000.
No.
Yes, YouTube and Google Accounts are the same... It tells you at the top when you sign in to YouTube!
You have to put your Google ID and password in it. It is synced with the Google accounts as a Google product.
Google accounts are useful for many reasons. Their e-mail has a large capacity for data, they have a drive section which is great for creating and sharing documents, graphs, and spreadsheets, and an instant message system that offers easy video and conference chatting.
Incognito window lets you sign in 2 different Google Accounts. It does not store history so can be used for that.
No. I think the only way Google Accounts expire is if Google shuts down or something. Files stay in your Drive until you delete them or until one of the servers that holds the documents crash or something.
You could sync Google accounts to Chrome. This can be done by the people's tab in the Settings.