New account

I want to create a second account. My goal is to start playing more varied openings and in time make my new account my primary and only account. Also I want to get rid of my current username.
It should be legal as stated be lichess live stream and other places. Multiple account are only discouraged - but do not violate any rule as long as you do it for a purpose.

How can I do it? Right now I am blocked by "This email address is already taken" when trying to create a new account.

1 email per account.

get a new mail

You have to use a different email for a different account. You can change your username here: lichess.org/account/username.
With "purpose" it is intended a valid reason like only berserk, only blindfold, only variants (not sure about this one).
If you instead want to just do the same things you are doing on this acc but on another acc, then I recommend closing this one and open a new one.

basically what Deadban said. There's no reason why you can't test out new openings on your current account.

Can I use my email again, if I close my current account?

How is it prevented that bad people makes lichess.com accounts with their "friends" emailadresses? -so that the friend can never make their own account.

Edit: Ok @Deadban already answered my question.

No, the email remains linked to the closed account in case you want to reopen it in future.

#6 If an account is registered but the confirmation link is never clicked, the account eventually disappears.