I suppose the elves might be hard-pressed for nourishment but if they're going to be running with the Dalish (who already have their own methods for getting food, and now you've widened their viable range for hunting and gathering exponentially ) then it probably won't be as foolproof a plan as you believe. Meanwhile you've likely got massive overcrowding in the cities, ideal conditions for disease, all while letting the elves and whoever else might decide to take advantage of this situation have free reign over the countryside. You mentioned only taking the "essential people", but what does that even mean? Is there even a real plan here? And what happens to the people who get left behind? And then of course, there is the entire process the mass harvest and the transportation of everything back into the main cities, which itself would be a great opportunity for the elves to use the eluvians to cause disruption and chaos.
EDIT: Not to mention if their little "webways" offer protection from the elements than the elves could very well be sitting pretty for the winter while the humans in their cities fester and die off.
The Dalish can feed their small clan , but they wouldn't be able to feed the city elves , they are mostly hunter ...They can't produce lots of food.
There's no way the whole city elf population can use the Eluvian , they are too many of them.They do not know the land much outside their alienage , they have no food , and no mean to produce some (most farms belong to humans) ,they don't know how to fight , they have no weapons.(I think they were throwing rocks or something at Halamshiral)And the Eluvian network is dangerous ...tombs , thin veil , dead ends etc...And I'm not sure they could grow food in the strange inbetween world.
All the humans have to do if the elves start causing a mess , it's simply purging the alienages.
The Dalish would have been a better choice , they move a lot , they have mages etc...they can hide the old and the young in the forest with a couple of hunters ...etc.
But again the only thing the elves can do is being a pain in the ass , and try to annoy humans until they are tired and willing to talk.
A real war is impossible.