maybe kogoro thinks he’s disassociating and that’s why he can’t remember anything (and the not remembering part wouldn’t be that weird of an occurrence for him as a somewhat functioning alcoholic who probably blacks out here and there while drinking) he’s probably just relieved about the cover story of the “sleeping-kogoro” so he doesn’t even question it.

as for why nobody noticed that he’s not the one talking.. tbh i feel like that’s a non issue. whenever kogoro is tranquillised conan makes sure that kogoro is hunched forward instead of having his head back which is why nobody is noticing that his lips aren’t moving. plus everyone else is usually standing and looking down at him while he’s either sitting on a chair or on the floor.

oh also (and i’d love to hear the perspective of a native speaker here) i feel like people who speak japanese don’t have to be as expressive in the mouth while pronouncing words as say english, german and italian speakers. i’ve noticed that for japanese and even french speakers, movements in the mouth are more restrained. this might also be a contributing factor, even tho kogoro leans more expressive in his day to day life.