Got an answer from nvidia:
Please take a look at Chapter 11 of the driver README:
The section on Vblank syncing has information on setting the
__GL_SYNC_DISPLAY_DEVICE environmental variable to specify to which
display device OpenGL should sync.
Hope this helps,
This does indeed solve things after some testing, but it's still a problem if four workarounds are needed for the whole not being able to set primary display issue; things should be easy, so that GNU/Linux appears welcoming for new users with nvidia cards and more than one monitor.