I have been using the "Remove" button. I'm going to say that currently items are being deleted but in prior versions I'm guessing they were not being completely removed as they still show in the pdf report. I have never used the write off button.
Deleting inventories always deleted them. However, in previous versions (a long time ago), there was just one option and it showed a modal asking you if you wanted to delete it completely or write them off.
If you have them marked as "write-off", you can delete them anyway if wished.
With the Export Pro add-on I know you can filter out what to export but could you possible tell me what fields are related to multi-inventory. I'm able to filter out the main parent product of multi-inventory items but not able to list the sub items. I would use the pdf report but it has the sub-items I had removed plus with only about 30-40 main items the report takes up 18 pages which is crazy.
If you want to export all the multi-inventories data, you should use the "ATUM Multi-Inventory" template and not the products template.