Interestingly the DL super I spoke with also thought this was a reasonable question...
You and I must have very different definitions of "reasonable question."
Long conversation with - DL supervisors today, and in short they accomodated passengers from the cancelled flight today.... so I asked to verify the status of those seated now, in the 12 seats previously empty.... and guess what? They would not say wheather they were SM, or paid F, only that "yes some DM and F passangers were accomodaited..... I demanded a specific answer and the bosses at DL declined.
IME, reasonable questions are answered quickly and easily and rarely require one to "demand a specific answer" after being denied an answer. Perhaps you've never seen the cards many phone operators have at their desks to deal with unreasonable customers. Start sentences with, "I agree with you completely..." or "You are correct..." but don't actually give in.
Your asking about passenger information is out of line, and your demanding it be provided crosses a line. DL is a business, or service provider, not your employee, child or even opposing party in a lawsuit. I'm not sure where you get that you are entitled to demand any information from them, especially over something as minor as an UG on a short flight for which you did not pay for F.
Here is some more unsolicited advice - here are your choices -
Choice One: You can continue to get upset, spend time fighting on the phone with DL, get upset and frustrated
and ultimately get an UG if one becomes available and maybe some points for your troubles, or,
Choice Two: You can accept that you didn't get an UG, calm down, send DL a letter/email expressing your disappointment,
and ultimately get an UG if one becomes available and maybe some points for your troubles.