I believe there are certain time frames (for example the Greenhill mission), where you cannot enter Rakutei mt. at all. Since this is a character recruiting mission, the area is unavailable when you are not supposed of getting characters, even if the mission is already completed.
You can enter Mt. Rakutei after recruiting Humphrey and Futch as long as a) You're not on a mission and b) you have Futch (I do believe that Humphrey is optional) in your party. So if you missed the plans you can go back and get them or if you want to do the Pixie Trick.
a bit oot here (can't find a thread specifically about the pixie trick), but does the pixie trick work on the two pixie variations (before and after matilda liberation) or does it only work on the later pixie version? I tried solo-leveling on spikers and stuff, but they only get me to about 77-80+. I haven't tried it on early pixies x6 'cause I can't find any, LOL.