Update :: The issue is resolved ! What finally worked seems silly. I read on some Apple forum , people reporting same issues with original Mac and external displays. One solution they suggested was to continously switch on/off the external monitor when its connected. This however did not work. So out of curiosity, I unplugged the DVI connector end on monitor ( HDMI - DVI cable) and reconnected and the display came back ! Some loose DVI singal-pin connector ? Surprising, as the dual-booted Windows was before properly displaying on this external monitor.
Lesson learned: Sometimes solutions to some crazy problems are simple enough - and we generally overlook. My mistake, I should have tried this before I wasted your and my time on thinking about complex solutions.
Thanks a lot RehabMan, for your assistance and time. Respect and appreciate the help you do for this community here.