So, they really don't know. In fact, the first one is a study for waste that was occurring prior to the new standards, which shows that a lot of food was already being wasted by children (the new standards went into effect in 2010, the research was done from 2007-2009, only the analyses of the data was done after the new standards went into effect). The second one didn't compare the waste, only selected to track how much fruit and vegetable waste there was, which means that we have no clue how that compared to waste of food from before. Perhaps they are eating more of their other food choices than before. Maybe the fruits and veggies are now the main thing being thrown away as compared to before. We don't really know, but the research from before shows that there was a lot of waste of school lunch before the new standards came about.
The other research is actually more interesting and doesn't show anything I'm not familiar with. Yes, children have to have some incentives to eat/try new foods usually, particularly veggies and fruits. I wouldn't quite go with actual monetary bribes for eating the food but think the tokens are a good idea, especially for trying the different foods.
As for how much, that first study you posted gives a small idea, at least for middle school students of how much was getting wasted.