Why an 18-inch battery breaks the G3 design

Could an F3-style gun arrangement let G3 carry 18-inch guns without changing the design's limits and performance?

No; the larger guns and every supporting system add too much weight to preserve G3's other characteristics.

Moving from G3's 16- or 16.5-inch battery to 18-inch weapons increases more than barrel weight. Shells, hoists, turret machinery, structure, and ammunition handling all grow with the caliber. Those penalties consume the margins needed to preserve protection, speed, and displacement. The answer also notes that the underlying G3 turret work retained roots in the 16.5-inch concept even when the ordered design moved toward 16-inch guns, leaving an upgrade path within that smaller scale but not to 18 inches.