Lucy's statement of "Dressing up as the polar opposite of your personality," is actually spot on. Given she's being incredibly nice by getting extra candy to give to Linus and acting the polar opposite of her usual self. She didn't dress up as an evil witch, just an ugly one.
Linus can see through the commercialization of Christmas because he thinks the mystery person who brings presents by night comes at Halloween.
When Linus says "if" (rather than "when") while talking about the Great Pumpkin appearing, that slip of the tongue can be seen as him subconscious means of providing an excuse for why the Great Pumpkin never appears.
Just how neglectful are Linus' and Lucy's parents, that Lucy has to be the one to check on Linus' room in the middle of the night, discover he's not there, and go out to the pumpkin patch to fetch him?
