For a guy who is just diving into the 70E you sure are asking good questions.
1st off, I dont see any HRC 3 for 600V MCC's, everything else, but not HRC 3. Are you looking at the 2009 70E?
Your logic makes sense, but it dosent fly for the 70E, there were a lot of proposals based around the concept you are discussing, all shot down.
In the tables, the highest HRC is determined for an assumed working distance, and the limitations of the notes. From there, the other tasks are all a "Gut instinct" of the commitee, based on probability of an event and proximity of the worker, the commitee uses the terms "Task based" or "Risk based"
When you do an analysis of the actual system, you calculate actual worst case Ei at a distance you choose. The commitee uses the term "Hazard based", risk or probability is not factored in. Lets say you did 2 studies for a given point, one at 18" (For T/s, removing covers, etc) and another at 36" (For visual inspections, IR scanning, etc) then you could do other tasks at 36" working distances at a reduced PPE level, but otherwise you would be just guessing.
I am part of a Hazard vs. Risk subcommitee that is looking closer at these issues and will be reporting to the 70E commitee with our findings.
Here is an example. At a major automotive plant, they required a 2nd person in the substation for CPR and safety observing for all switching tasks. The arc flash boundary was over 20' so the other person needed to be outside the room or in HRC 4 PPE (As calculated at 24" on thier study and labels). So we calculated the distance that the Ei was <4 cal/cm2 so that the safety person could observe the worker wearing only HRC 1 clothing and safety glasses (This was before the facesheild was added to HRC 1 in 2009), that ended up being around 8-10'. 2 labels were put on the equipment.
Does that help clarify?