I would say that to "presume the good will of the objector" does not mean that you actually need to believe that that presumption accords with the truth. But to act in accordance with that presumption is necessary if there is to be any chance of a useful discussion. To do so is a willed action, rather than a reflection of opinion.

I'm in danger here of diverting this thread into a philosophical discussion which doesn't really belong here.

Out in the big world the Windrush episode appears to be having some significant developments. It's fascinating to see how Amber Rudd is trying to reposition herself as somehow not being herself responsible for the illegal actions which she condemns, and ostensively presents her self as correcting. (We'll see...)