C172 Flight Instructor reported difficulty maintaining cloud clearance requirements while on a scenic tour and may have descended too close to buildings over a populated area.

Narrative

I was the Pilot Flying (PF) doing an instructional flight with a student up front and another CFI (who recently lost their medical so cannot be PIC) in the backseat just riding along. We were doing night flight training and my student wanted to do the river tour; where we fly along the river and get a good view of the city. The student asked if I could fly it so that they could look outside; which I accepted. I had done the tour many times. While flying up the river; we planned to go at 2500 ft. MSL; which would keep us above all buildings by well over 1000 ft. However; as we progressed down the river; the clouds got lower and lower (not forecasted); while they were initially above 3000 ft. MSL. With bravo airspace to my left and ZZZ1 delta airspace to my right; and with little time to get a clearance into either; my best option was to continue forward over the river at a lower altitude. Since I was fairly busy and the CFI in the back was following along on their sectional; I asked my CFI friend in the back what the highest building was in downtown ZZZ along the river that we hadn't yet passed; they said 1000 ft. MSL. So; I descended down to 2300 ft; and then 2100 ft. MSL for cloud clearance. However; right after we passed downtown ZZZ; my friend told me that in fact; there was a building they missed at 1300 ft. MSL. As a result; I realized (too late; obviously) that I had inadvertently violated 91.119; which requires that I fly at least 1000 ft. above all obstructions within a 2000 ft. radius. Fortunately; I was still 800 ft. above the building; and I was over the river; not directly over the building; so it was not a dangerous situation in this case. In fact; I may have been 2000 ft. away from the building laterally; in which case it would not be in violation of the regulation. However; it is easy to imagine a mistake like that resulting in a worse scenario; and it was certainly a mistake on my part regardless of if it was a violation. I was ultimately the PIC of the flight. No matter how experienced I am; and no matter how experienced my CFI friend helping out in the back was; the operation and safety of flight ultimately comes down to me. I relied on my friend in the back for something that I should have done myself. For that; I am to blame and I have learned a valuable lesson. I'm the PIC - it's on me to verify that sort of stuff. I will not make a similar mistake again in the future.