I had Persuasion 8 (which is the maximum Persuasion I could intrinsically have), and I needed to reach a Diplomacy 28 check. The charisma spell/potion/headband don't stack with each other, so I used the +4 CHA headband that gave me +2 Persuasion for a new Persuasion score of 10. So I needed to roll an 18 to reach 28. I do not see what else was there to use.
I recruited Tsanna to see what she is like (I know, I know.... NOW I know), the sick old man on the bridge who asked for berries died, and that guinea pig woman died during the surgery. I think I happened to do extremely badly on the very things that mattered for this check, and that is why I got such a difficult check.
Note that the 28 check was followed by a 29 check, and finally there was a 35 check. Naturally, I only succeeded in the 1st one (after a lot of reloading), yet they did not lynch me. There was also 1 dude in the whole crowd who was standing up for me, so at least I had done 1 thing right!