How do C. elegans manage nutrition?
The answers are:


Yes, they know when to stop eating. See this paper.
Yes, they store extra energy as fat in the intestine.
This one is a little more complicated. They can use extra energy, but not to move faster. Movement doesn't take much energy, and until they're close to death from starvation, they continue to move quite well. As for eggs, they do indeed slow down the rate of embryo production when nutrition is inadequate. But the total number of eggs is limited by the supply of sperm, so there is an upper limit on how many they can produce, even in unlimited food. Probably the main benefit of stored nutrients is to allow them to survive longer if the food runs out. They are very, very good at dealing with a variable food supply -- much better than you, for instance -- and they do it partly by storing up extra energy. This paper and references therein describe some starvation responses. You can also look up L1 diapause, the dauer larva, and adult reproductive diapause.