As far as I know, when you’ve started SL a mission should pop up, once completed shadow’s have a random chance to show up. The missions show up as ? Shadow something has a few prefix’s that pop up. I don’t remember if you need a shadow lab character or not. I don’t think you do. I also don’t remember if you can reset the shadow lords quests itself or just character specific ones.
I don’t know of any difficulty completion requirements either. Do you get shadow missions at all? I can show a pic when I get home of what one looks like.
@TotalJimkata I don’t remember the quest name itself since it was ages ago but maybe offer some insight?