Oh gosh, I haven't used Warrior Lilith since forever. To my knowledge, Magma Shield shouldn't break Sleep anymore (unless the change was exclusive to Guy Fawkes which is unlikely). Even so, Warrior Lilith would break Sleep regardless of the fact since her attacks don't bypass the effect in general.
As for why Sleep was chosen as a trigger, I believe one idea was that there haven't been very many Eddies to date that support Sleep teams. In recent days, there have been a fair number of new allies that bypass Sleep, but not many Eddies to go along with them. It's also worth considering the thematic element of plotting to sneakily set up bombs while your opponent sleeps.
Another fun fact for you all - elements of his abilities, such as Magma Shield and Ghost, are reflective of the real life history of Guy Fawkes - how people have burned effigies resembling him in an influence that has lasted centuries.