Hi,
Workflow is not provided in Movement Journal out of the box. I guess you have developed this.
1. You have to create task element in workflow configuration. So that only the user who assigned to that record will be able to edit.
2. It depends on the way that you did customizations. If you or your developer were added this button and functionality to that form, you would be use request change functionality. So you have to develop this and add required buttons.
3.This topic is depends on your customization too. You have to manage workflowstatus field in InventJournalTable by methods like init,insert, update, validefield and cansubmittoworkflow.
Consequently ıt would be good to review your workflow customization and improve it according to your needs.
Best regards,
Sefa