Yes, you can use multiple Google accounts.
no
It's free with a Google account. Google accounts are also free.
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.
Incognito window lets you sign in 2 different Google Accounts. It does not store history so can be used for that.
Google is very useful because it finds webpages for you.
Google offers a multiple account log-in option which will allow you to have 10 accounts open in one browser at the same time. You can also open a google business account for $5 per person, per month. Google has several options for having your own account/site.
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.