If for some reason the extra bit of food doesn't help (though I suspect it will ) I was going to suggest that you block the cat door from the outside. Maybe keep a rubbermaid container or cardboard box with something heavy in it and push it up against the door so a certain naughty 'somebody' can't get through it (providing of course that you have another door that you can use to get back into your house!)
And then maybe she and Ellie can be naughty together and hopefully keep you out of it.