Would single 12-inch wing guns have fixed the Dantons?

How much better would the Danton class have been if its twin 9.4-inch mounts were replaced by single 12-inch guns?

Not much; the change would have produced a seven-gun broadside without turning the design into a satisfactory dreadnought.

Replacing the secondary mounts with single 12-inch guns might have allowed the Dantons to claim an all-big-gun label on paper. The resulting seven-gun broadside would still have been awkward and would not have made the ships convincing dreadnoughts. The deeper problem was the overall arrangement rather than the diameter of individual guns.