Ah! Good conversation is so lacking these days!
You make some good points. Ultimately, however, I must fall back to Tolkien's vision of elves, which is not only the basis for our fantasy elves, but also answers these questions quite well.
Basically, Elves have renounced their passions. They do not seem sex or money or power. And that is why their numbers are lower than might otherwise be expected for a race that does not die of old age. I would seem that adrenaline, or other chemical/spiritual/mental changes caused by a state of arousal cause elves to lose their immortality. See: Arwen, after falling in love.
That being said, DF elves are CERTAINLY not Tolkien elves, but of course, should strive to be as realistic. The problem with elves is absolutely not Tolkien's vision, but others interpretation of it. DF elves could simply not exist! They are uber-passionate, super-psycho, and breed normally while being un-dying. These could not coexist in a living being that did not immediately overrun it's environment and kill itself off. No help from warring humans required. If DF's elves were actually required to eat meat, being cannibals, as they seem to be, and did not recieve norishment from anywhere else, they would DIE. Add [carnivore] to their creature file and remove [outdoor_farming] from the entity. They'll die in year 4.
Even if they could gain nourishment from the bodies of their slain enemies, the caloric intake they would gain would be VASTLY overshadowed by the expenditure of energy required to kill said foe, and further made a moot point by the numbers of fallen dead surrounding them. Since they would not eat their own kind, their kinfellow's meat would be wasted, therefore leaving less biomass for the next generation, every time they attack.
What does this all mean?
MY elves have been modded not to be cannabalistic, and to age very very slowly. You might also simulate their "lack of passion" by making a third caste of elves called "breeders", named "female" and making up 1/100th the population, and changing the regular "female" elves to not actually breed by removing the "[female]" tag.