> "[T]he schema for schemas does not allow [minOccurs or maxOccurs] on the
> child of <group> when it is named."

Unfortunately XMLSpy doesn't flag this.

... but looking at the last sentence of the paragraph:
"This in turn is because the {min occurs} and {max occurs} of the particles
which refer to the definition are what count."

So I assume it's ok to relocate the min and max:
<xsd:group ref="additionalElements" minOccurs="0" maxOccurs="unbounded"/>