Can we have a non-driven wheel in contact with the floor in this year’s project, for the sole purpose of determining actual speed and distance for the robot?
Yes it can. There have been many references to this exact question both on CD and through the official Q&A. As long as no additional traction is gained, your scenario is perfectly legal.
This message will likely be followed by a litany of “search before you post” posts, as per Internet requirements. Don’t be offended; just friendly people trying to help.