subdomains, domain names, & domain mapping with multi-domain

I have mainnetworkdomain1.com as the main domain.

Also there is domain2.com, domain3.com, domain4.com, etc.

I am able to make subdomain1.domain2.com, and basically anysubdomain.anydomainonnetwork.com without issues.

Is it possible to create domain2.com without a subdomain? Would I need to create subdomain.mainnetworkdomain.com and point domain2.com to that mainnetworkdomain.com subdomain?

Can we create domain2.com and http://www.domain2.com and in addition still add subdomain1.domain2.com, subdomain2.domain2.com?

Worded another way – once we enable multi-domain do the sub-sites on the WPMU network need to have a subdomain unless it’s mainnetworkdomain.com? I.E. Additional domains must be a subdomain on the network or a domain.com pointing to a subdomain on the network?

Happy Monday!

– David Monk Sr