I think the title "vehicle commander" should be "assistant or co-driver" for as driver you are solely responsible for the vehicle and driving.
The DVLA site mentions "vehicles that need a second person to help with observation, such as military vehicles". Not all military vehicles need that assistance, in fact most don't.
As I said previously, my experience was DVLA determining that a 432 did not need a second person and was appropriate. This may have changed and it is always good to ask, DVLA will talk to you.
Can any person be deemed an observer (or whatever you wish to call the second person), or is a qualification required such as an H licence...
I would say that if you take reasonable precaution, mirrors properly fitted and adjusted, maybe with a rear view camera or whatever and drive with care and consideration. There would be no offence. Speak to a traffic cop if you can and just ask their opinion.
I suppose worst case scenario, if there is an accident, no matter who is to blame, how would things go ?
Ask the relevant authorities the questions you need answered, we can only tell of our own experience and offer opinion and as you know "everyone has one."