Is anyone able to tell me if there's a categoric way of proving who has completed an action?  We have a record where an action was completed on a particular date, and the properties for the action say that it was last amended on the same date by someone.  Are these one and the same?

The properties for the action will tell you when the last change was made, and who did it. However, if Person A does something bad to a record, and then Person B goes into the same record and changes something unrelated to the "bad" change, Person B's information will persist when you report on the date last changed.

There is currently no way in RE to track changes over time, or to associate a specific change with a specific user if it's not the last change made. It only records the timestamp and user ID of the last change made.

If you had a database backup from the period between Person A's changes and Person B's, then you could restore that database to a test environment and then see who the last changed user was there, but that's the only thing I can think of.