The problem with using your last protein source is that if it doesn't work or it only works for awhile, then you will have nothing to go to. Have you tried duck, venison, and rabbit or not yet?
Fish has helped Ellie's itchies it seems, but maybe not so much anymore. It is frustrating.
And the hydrolyzed diets don't work for her. Z/D Ultra is supposed to be a good one if you want to give it a try. I also think that since the problem hasn't gotten any better with so many changes, it may be time to talk to a nutritionist.