What is the correct way to map multiple domains in a WordPress 4.1 multisite install?

I’m aware of this question and others but have not found a definitive answer to how a domain should be set up with multisite so that www and non-www variants behave correctly. I believe I have wildcards set up correctly at the DNS level and the suggested solution to add define( 'NOBLOGREDIRECT', 'http://www.example.com' ); works for my main site but not other networks in the same install.

How to use same email for multiple users

I have a very big multisite. And I got a request to enable option that multiple users can use the same email. I found a plugin “Allow Multiple Accounts” which doesn’t work properly. I should figure out some other solution for that. I know that I could use something like adding +sometext to every email, so it will show different to WordPress. Do you have some other solution, that can be done here?