Give her time.
I got my EE from the feed store with another EE, a BR, and a RIR on March 20th. The BR is looking like a roo and the RIR died, but that's not the point right now. When we got them, the EE was just the same as the others. Right now, she's grown a bit but barely at all. The other EE is feathered out, including the head. The BR is getting most of his head covered, the back of the head is all that's fluffy now. This little EE feathered sloooooowly. So slooooowly. She finally grew pins on her back about a week ago, before that I started to notice her tail getting a bit longer. Now she has a small "fan" of feathers on her back. Yes, feathers! Not pins, feathers! A couple days ago, I noticed pins on her head. Yay!!! Today, I saw that the pins on her head were turning into feathers, spreading out. They stood out because being an average EE, she had a chestnut head that feathers in gold.
So hold tight, she will likely pull through.
Oh yeah, I forgot to mention. I'm in the South Bay of CA. The three are living out in my yard in an unheated coop. I think Nan will be fine in the coop as long as the others don't use the roost; they can snuggle up together. She might be able to fly a short ways up, so maybe you could make her a ramp or ladder?