Solved!!
I talked with a support person named Kent at Flexbits (fantastical)
He suggested fooling with this file to fix...
/System/Library/Frameworks/CalendarStore.framework/Versions/A/Resources/Mail.applescript
because I don't know code, I just moved it out of the directory, but still no fix. BUT, inside the same folder is a "scripts" folder with mail.scpt in it. I moved that file too and all is as it should be.
no calendar.app opening mail.app
not sure of repercussions, but I can always put it back.
My number one pet peeve from Snow Leopard is now back in Mountain Lion. I don't use OS X Mail. Something is triggering it to automatically open up... all the freaking time... sometimes it asks me to set up an account. Sometimes I think it's something from my calendar that triggers it. Any way to stop this from happening?