Hello!
This game is just frustrating, and I'm all but convinced this achievement is bugged, at least on PS4. I've done four playthroughs making very obvious mistakes, and still haven't managed it. This game isn't particularly complex; it shouldn't be this difficult to achieve (in fact, the only reason I know 8 mistakes are needed is because of the XB achievements - have you read the descriptions for the PS4 ones?)
I listed the Churston murder one as #3 on my list there. I also said Wang Chung (or whatever his name was) and Ginger Cat, and nothing. Someone suggested trying Donbury and London, but those are lesser mistakes than saying Wang Chung and Ginger Cat, and are technically correct answers all things considered.
Given that the only person on PSN to get this achievement got it when giving the gun to Hastings - AKA the last point in the game where choices seem to matter - I'm willing to bet that's the last trigger. I tried giving all the wrong answers during the final group meeting, but it refuses to let you go through with any of them unless it's the correct answer.
I've been told that the "bad" way to question the final suspect is by using both the cough syrup and the sedatives instead of just the sedatives. I did this, and just ended up getting the Red Cross achievement either way, so that doesn't seem to count, either.
Again, unless it's bugged.