Lettuce has no value at all other than moisture, Herbivourous/omnivorous herps can eat brocoli but it should be given very sparingly. ditto spinach.
asian veg are great, buk choy, choy sum etc, endives, kale, the list goes on.
No, not all wrong, carrot is ok, not much goodness in potato peel, excepting you feed it to a worm farm and feed the cham the earthworms. Half an apple, pear, orange even, rockmelon, mango ...variety is key. different foods contain diff levels of different vitamins and nutrients, so the aim is to cover as many as possible.
feeding healthily gutloaded prey makes for healthy reptile. Folk rely heavily (too heavily imho) on suppliments but they are only that, supplimenting.
That dosent mean dont use them, but create a healthy diet to begin with.
grass? hmm, if you do throw in some grass on occasion, make sure its fresh deep green stuff, never mowed clippings or grass tht may be contaiminated by pesticides or lawnmower fuel residues. No idea of the nutritional value though.