3 answers
I either use Data Loader or Workbench, yes. I'm not sure if they're the same, I've never compared them.
I usually go into the permission set and pull the ID from the URL and update the PermissionSetAssignment object. I delete all IDs that have the ID I pulled as the PermissionSetID.
However, I recently noticed an option to Manage Assignments in my sandbox - that's actually the fastest route. You can select all and click Remove Assignments.
