I use a boat produce called "Ducky" to remove hard water spots from my boat when they have baked in the sun on the Colorado river,and have always had good results.It is available at any marine shop.But I don't know with the arsenal you have already tried,maybe this won't even work.
http://www.duckyproducts.com/water_spot_remover.php
It may be time for a high end detail shop to acid wash it and polish it out,
I have seen this in rare cases be very successful