Not sure I understand the results that are being described. Ordered quantities as tracked by default ZC are in no way touched or addressed by this plugin. That data is collected from the orders table and there is currently no facility to write to that table or any related table.
Now on issues with user defined fields, I was going to look at how the code responds if the user defined field is present, but the field is not identified in the import file just to validate that the row data for that field is not modified, but there is no control if the field is present in the import file and the user has decided to leave the field blank. In that situation the code would work as expected by updating the record to be blank as it is/was in the import file.
So, could use a little more clarification of the perceived issue in order to provide a solution either by code or direction.
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