Why can yeast only use DAP in the early part of the fermentation process? Presumably we want to assist the yeast to reproduce during the early days after we pitch the yeast. In other words, we are willing to allow the yeast to expend MUCH of their time and energy in reproduction. They are going to be converting sugar into alcohol too but much of their energy is going to be used in dividing and reproducing. When the colony is large enough we want to inhibit reproduction and get the yeast to expend their energy in transforming the sugar into alcohol. My understanding is that if WE provide the phosphates later in the process the yeast will continue to expend more energy in reproduction than on eating (converting sugar to alcohol) . If we don't provide the phosphates then they don't have the nutrients they need to divide. In other words, it's not that the yeasts can't use DAP later. It's that we don't want them to. That's my understanding but I am not a chemist or microbiologist.... but you seem to have a different understanding.Diammonium phosphate, it's a nitrogen source for yeast but they can only use it in the first third or so of fermentation.
You can indeed, anything not consumed by the yeast will remain as food for any spoilage organisms that may find there way in and survive the alcohol, you could even leave enough to taste but that would be using a lot.