Hello all. SuperBee, Scott and all others; THANKS A BUNCH for all the suggestions. Here's my update (and I know you will find the "fix" impossible to believe).
As fate would have it, the day I posted about the spots, my car got blasted YET AGAIN with a sprinkler; this time in an entirely different spot of the lot and one that I was assured gets watered late at night. Anyway, here goes.
This was once again a case of Autopia coming to the rescue of a desparate member. Even before reading Tortoise's post above, my search yielded the DG WSR as an option. Quick call to the nearest CQ store resulted in a hit so I went and bought it.
WSR smells a lot like a water/vinegar mix. After washing the Z with ONR and drying, I scrubbed the hell out of the water spots with WSR and it removed all the fresher water spots I had received that same day (which too, had baked and hardened in the sun all day) but it did NOT remove the stains from the day before, the stubborn ones I created this thread for and that I had already tried all my polishes on.
Tired, disheartened, and out of time, I simply QD'd the Z and called it a night.
Left on my long road trip the next morning. It was sunny all three days and every time I walked back to the car over the next 3 days, the massive spots were staring me in the face.
On the 4th day while driving back to Denver, I got hit by a series of strong thunderstorms off and on for approx 5 hours or so. Got home and left the Z in the garage to dry off. Next day (Sunday morning), I went back to continue the battle. Once again, I used a light mix of ONR to clean off a spot (top of the spoiler) to assess the damage and.....SAW NOTHING. No water spots were there. Suprised, I washed off the entire hatch and again, saw no spots underneath! I pulled the car out into the sun and still no spots!
Needless to say, my Z was somehow cured!
So what gives? I am baffled by this. The only liquid that touched my car after leaving Denver 4 days earlier, was the rain from the previous day. Right up to the moment I sat in the car to begin my journey back to Denver on Saturday morning, all the old spots WERE there. By the time I got home, they somehow disappeared. It has to be the rain (I told you, you won't believe the "fix"). May be it was the sheer length and intensity of it that forced the spots off. May be the residual WSR somehow got reactivated by the rain and nudged the spots off. May be there was a midget on rollerblades hanging onto the back off the car the entire way back, polishing them off. May be....?
It is unexplainable. All I know is that I couldn't be happier! That day, after drying the car off, I washed wheels, tires, and wheel wells, did a simple AW job on the entire car, including wheels, dressed the tires, and came back into the house with a sense of utmost relief and jubilation.
As I said, I am still in awe. Would love to hear your thoughts.