Hi,
If you remove first and second Value from below xml, z_elements multi value lookup in instance will have "IDEA00649" value only. IDEA00529 and IDEA00537 will be removed.
But you remove all Values in xml, z_elements multi value lookup in instance will not be updated.
<ColumnValue name="z_elements">
<Value>IDEA00529</Value>
<Value>IDEA00537</Value>
<Value>IDEA00649</Value>
</ColumnValue>
I think that at least one Value tag is required for xog-in instance.
I have following idea. It may not fit your requirement.
For example, adding extra value like as "Nothing Selected" in lookup.
Note: You need to assume it is for empty lookup value.
If you would like to remove all values, you will use below xml.
<ColumnValue name="z_elements">
<Value>Nothing Selected</Value>
</ColumnValue>