Very interesting work!
I think there is no strong "no parallel lines to paper's edge" rule; it is just something to be aware of. The concept of Sarah Vandam is strong and had an horizontal line.
In terms of your current composition, I would say:
- maybe try not to cut-off the mouse or its head (the cover is covering it now), so it has a readable silhouette
- maybe use stronger lights and shadows (e.g. a night lighting with warm yellow vs blue) to make the scene a bit more dramatic and guiding the eyes to the mouse and elephant facial expression.
- maybe use more lines to guide the eyes to the mouse
- maybe make it a one page spread to strenghen the verticallity of the composition (mouse is under the elephant).
can't wait to see where you will be taking this illustration !