You're over thinking all this. Stop. Take a DEEP breath :)
Okay. The way the mapping plugin works is REALY straightforward and simple: It says "http://domain2.com will pull data from http://subsite.maindomain.com"
Hold on to that for a second.
When you say you want to use http://www.domain2.com then you have to make sure of two things:
1) http://www.domain2.com works at all - If that, WITHOUT WordPress Domain Mapping, does not redirect to domain2.com then the issue is outside of WP.
2) You map http://www.domain2.com in the WP dashboard to point to your desired subsite.
Because in my missing www scenario, I get redirected to the registrar page for a parked domain.
That's not even pointing AT where you installed WP, and is totally outside of WP.
www CNAME = the registrar's auto redirect
And that's what was not right :)