Thanks that worked, I went back and fixed it. The guide says to threaten to break up to get her to go, never says to try and convince, give up, remove inhibitions, stay at stables, make her more willing to go, end spell, then reconvince her to go.You pressured Nell too much to go meet the fairy queen. You need to go back to a save before then or during that conversation and invade mind again and loosen her concerns before you ask her about the visit.
Now Day 64 goes smoothly.