What I like about approach #2 is that it bundles the configuration related to executable validation in one element.
Maybe an attribute could be used for the flag:
<executable-validation enabled="true|false">
<default-validated-executables>
<executable-type>NONE</executable-type>
</default-validated-executables>
</executable-validation>
Generally I like "positive" flag names better than negated ones (i.e. "enabled" instead of "disabled").
If you want to stick to two elements instead of an attribute, a xs:choice could be used to express that only one of the elements may be present in a given instance document.
> Would isValidationExecutionDisabled() not a better name?
Or isValidationOnExecutionEnabled()?