There really was only "lots of sugar" in Eddie's brownies.
It could be that Mark is so weird on his own that he doesn't even need drugs to see himself in a Gwar video. Lucas has a anti-social disorder
Makes sense. His in film back story is that he was a bed wetter past the years it might be normal, and was otherwise generally a 'bad' kid whose mother gave him up because of his issues. We don't learn what they are but they sound behavioural. Joe took him out of foster care and it's implied he was doing well enough under Joe's guiding hand (being the night manager at the time of the film) but when his bubble of existence is threatened he resorts to something impulsive, and then doesn't seem to recognise the negative fall out of his actions, calmly asserting things will just 'work out'. Lucas believes the store has magical powers
Several remarks he makes seem to imply that Lucas believes that the store is in some way magical, and benevolently so at that. Notably, his claim to be "guided by a force much greater than luck", and his assertion that nothing bad would happen to Debra now that she was "in the store." Corey got a rejection letter from Harvard
She's the only one who reads the letter, and takes quite a while to say she got in. It's most likely the shock and relief of hitting her goals, but the letter is in a very VERY small envelope and American colleges send successful candidates a huge pack of materials to get them ready for moving in. Assuming that's the case, the strain of keeping up the pretence throughout the film (and the inevitable discovery of her lie) would go quite some way to explain why she's so desperate for the ecounter with Rex to go right; and why she brutally lambastes her best friend in the immediate aftermath.