I must say I like the idea about the Snowy Mountain abyss part: we could see the SUM, with Little J in the bag, jumping down the corners of the abyss in a frog-like manner & Big J standing near the edge, unable to follow.

Then, when mouse gives her the gravity potion, she could use it to jump down the corners of the abyss, making her way by shifting gravity depending on what ledge is above or beneath her & how far is that ledge (if it's less then few seconds of flight). We could also have her being hurt if she finds no ledge & falls back down.

Also it would be a good thing if she reaches the dungeons from the bottom of the abyss & have two J's start dungeon level from the opposite sides. We can have sister at the bottom of the dungeon & brother at the top, in the dungeon tower.