The COLREGS wording is probably not as good as it could be. It is certainly causing a lot of confusion among this group and some professionals as well. The word "exceptional" means problems with the vessel, not the captain or qualified watch stander. Rather than (N)ot (U)nder (C)ommand think along the lines of "Not Command-able" For example a steering failure. See my earlier post #11 for a link that clearly explains the definition of NUC. That explanation matches what I was taught by instructors holding "Master Unlimited" credentials.
Unfortunately for those of us who must from time to time leave the wheel unattended there are no lights or day shapes defined in the COLREGS to be displayed to convey that status to nearby vessels.
I can't imagine any legal troubles for the capt of a recreational boat coming from simply displaying the NUC lights or shapes improperly. UNLESS there is an incident. Then the improper display will only add to your misery.
I did read your post #11 and the comments are not part of the reg itself but one sources interpretation of the word "exceptional." Now, I interpret it's intent in the same way you do, but that's just an interpretation of something not defined in the regs. Many of us have been taught exactly as you were, but it's still an interpretation, not anything defined in the regs.
The situation of no one at the helm isn't covered because the colregs do not consider that to be acceptable. Therefore, I could argue if I chose that it's "exceptional." Regardless there is no other provision so those people pick what they feel comes closest.
Now, if I'm on the seas, I'd rather see NUC if no captain than to see nothing displayed. Also, as I look at rule 18, I find treating the missing captain as NUC to be good as it means I have a responsibility to keep out of that boat's way.
We need to not get so hung up on the specific words or interpretations we've been taught that we lose sight of what may actually be best in circumstances. As to liability, a boat without anyone at the helm is likely to incur some in an accident. However, using the two red lights won't increase that. In fact, it can increase the liability for the other boat even if some feel NUC was misused. At least a warning was given.
Psneeld has lived with the rules, but still sees the value of expanding simply because nothing else exists. I'm on record as opposing single handed ocean crossing where the only helmsperson is asleep at times. However, if one is going to slow the boat and go below and sleep a few hours, I'd rather see the two lights signifying NUC than to keep waiting for the boat to take some avoidance action that's not going to come.
I can't see any incident in which signaling NUC because no one was at the helm, would increase the liability. It's a warning to other boats. Is someone going to go after them for a false warning? The liability comes from no one being at the helm, not from red over red.