issa > ilza?
Outside of the "there is..." construction, ilagon mainly means "to lie", so I'm not sure it would be used in a question like this. But it may well be.
Sure. I guess I'd gotten the impression that sagon/ilagon represent a pair like Spanish ser/estar — but I acknowledge that could be a leap.
I myself do not fully understand this yet, but apparently with skoriot, either sagon or ilagon may be used. See here for more.