iOS - Mail auto-fetch will not disable
After several weeks of testing - this doesn't work. All it does is prevent the badges, not the actual unwanted mail fetch
What appears to fix this is to turn off Notifications for Mail
Settings > Notifications > Mail > Allow Notifications.
This does mean that you never see Badges if/when any mail at all is received, so you do have to actually launch Mail to see if it worked… which then loads manually, of course.