It appears that melting of polar ice sheets will have a far greater impact on future sea level than anticipated

No. You are over thinking this. Its just liquid+liquid. I don't understand how so many different people can explain this simple concept to you over and over and you still don't get it. Your ice cubes in the glass analogy doesn't work because the ice cubs are not in the glass to start with.

The ocean has a finite volume. The polar icecaps are not currently part of that volume. They are on land. Ok? Do you get it? The current volume of the ocean does not include the water stored in the icecaps. Get it? As the icecaps melt, their water will run off into the ocean, thus increasing the volume of the ocean from what it currently is. Please just get it.

Side note, do you really think that the world's scientists don't know that ice is less dense than liquid water?

/r/science Thread Parent Link - nature.com