I believe that you are mixing things up just a little.
greenland sits on top of magma.
magma is supporting greenland.
if greenland becomes lighter then the supporting magma will push greenland upwards slightly.
it is not only that the water weight on the ocean floor will cause greenland to rise it is that greenland has become lighter , the ocean is vast and water seeks its own level so the small amount of rise in greenland due to the water released from the melting ice into the oceans will be
relatively miniscule compared to the rise from
the upheaval of the supportive or load bearing structure in this case the magma under greenland.
heres a simple experiment to show what will happen.
get a large bowl.
and a large balloon.
stretch the balloon over the bowl.
place a single ice cube in the center.
pour water on top of the balloon.
let greenland melt...
what happened?
I believe that the balloon will push upwards.
even if the water is removed as it melts.
