@AlexOnTheHill thanks for your post.
Once a timesheet value/entry has been created, it can be amended by it's owner, or any user that has visibility of the owner's timesheet. This is controlled by the visibility area in the admin section.
As you say there is a full audit history that is read only which captures any changes to the individual timesheet value/entry.
There is no current way to 'lock' completed entries, however we've had a change in our backlog for a long time to introduce the concept of timesheet approvals. This is particularly important in project management for example.
There is no limit on the number of categories that can be created.
Happy to answer any further questions you have.
Alex