@Justin Xu,
A "chronic" dieter is someone who is always on a diet. They lose weight, quit dieting, gain weight, go on a diet, lose weight, quit dieting, gain weight, and on and on and on.
Wearing a shirt "out" (untucked) is like wrapping a towel around your waist in that you're trying to hide your stomach. Laying down to zip up your pants is what some overweight people do to help them get their pants when they don't fit right anymore.
The towel, shirt and zipping are all ways that overweight people try to hide or deal with their weight.
I think you misread the "4801 bs" -- it should be 480 lbs. lbs is the abbreviation for pounds.
"Skepticism waiting in the wings" means that she wants to believe that she can eat lasagna but she knows it isn't commonly considered a diet food. She's
ready to be skeptical.
Waiting in the wings is a term used in theaters where an actor is about to go on stage and is waiting backstage for their cue.