Hi MaxB
To filter by several suppliers, you'd need to use de IN clause and enter values separated by a comma.

In variable products, suppliers are linked to the variations, so these variations should be included in the export. What will never be included are the variable parent products.
Are you sure this isn't the case? If so, please open a private ticket mentioning this thread and attach the CSV file and the template you're using if it's the case.
You can read how to attach a template in Ticksy here.
Best Regards,
José Andrés