How to set permissions on a Tasks List and also store a digital signature against the Task completer?

I am trying to create a Workflow from a Task List that I have. The list admin has the ability to send every individual (listed as an “approver” on the list), an email asking them to fill out a specific column of the task.

Once they have filled it out, the admin should then be able lock the list with everyone’s answers. Is a Workflow necessary for this? Specifically, the emailed users should only be able to make changes to their assigned tasks.

Also, I’d need a digital signature to be attached to their changes showing that they were the ones who made the changes.