3. Debug the ID sync is implemented fine. Customer IDs are case sensitive, make sure they maintain the same case when being ID synced and uploaded in Inbound files.
You can also check the trait graph of the Data Source Synced trait for which you have been uploading inbound files. Go to Traits > All Traits > Search with the same name as that of data source. You will see an auto created trait for that data source. Check if it has some devices as well as Customer IDs.