The main attribute of the Grissom
would be unrelated to her apparent age, familial relations or such: she was simply expected to look unlike a starship
so that she could be an all-new thing, a helpless survey ship that can trivially be destroyed by a Klingon ship that in turn stands no chance against the Enterprise
. This would not have been easily achieved by creating a ship with any commonality with the starship designs already seen in the movies - but OTOH the Federation identity needed to be made clear so a saucer plus nacelles were called for nevertheless.
So the study models were made, and the Excelsior
evolved towards as traditional a starship as possible, while the Grissom
supposedly fluctuated from oddball shape to oddball shape. But study models of the latter are mere rumors at this time, alas...