I'm going after the "Madman" achievement which entails completing the game on "Dark" difficulty. I accidentally changed the difficulty from Dark to Normal. I then loaded an autosave from earlier thinking that maybe the difficulty would be back at Dark, and it was! If I keep playing from that save, will I still get the achievement? I'm just not sure if somehow it will still recognize that I went to normal, and I would rather not play through on Dark mode and not get the achievement. Thanks.
if you played on darkmode and saved, like you said, you should be fine. As long as in your savegame you didn't switch difficulties then you shouldn't worry.