Sugar, Spice and One for All
You still have not explained how we can justify Jumbao's lack of Ghost type moves (sans Shadow Ball) and flavor moves for ghost typesI'll do one more post to try to convince people to at least consider Grass/Ghost.
I think that this typing would allow for a more interesting design for the preevolution. If we keep Grass/Fairy, the prevo will probably end up as basically just a small version of Jumbao. Not that that's necessarily a bad thing, it's just not that interesting IMO.
The typing would fit really well with the lore behind Jumbao's design. The root rope thing around its waist is a "Shimenawa" which, according to the artist is "hung around sacred objects like trees or rocks and wards off evil spirits that can inhabit them". So the lore given for Jumbao's design specifically mentions that spirits can inhabit trees and that the Shimenawa is placed around it to ward them off. So the prevo could be inhabited by an evil spirit and that is why the Shimenawa was placed around it in the first place.
And lastly, I think the ghost typing would reinforce the fairy typing of Jumbao. Jumbao's design doesn't really scream "fairy" to me. Fairy seems like a sort of opposite to the ghost type. Obviously there's the whole Clefable - Gengar connection, but beside that, ghosts feel more malevolent, sinister, evil, and corrupt while fairies are more carefree, malevolent, good, and pure. I think a good way to draw those characteristics out of Jumbao would be to contrast it with its prevo. The fairy typing would make more sense because the evolution from pre-Jumbao to Jumbao would represent a purification and transition from an evil spirit to something that is good natured.
TL;DR: Grass/Ghost is more interesting, it fits the lore very well, and it reinforces the fairy typing of Jumbao.