Most horizontal Brain Cells will fit in both bags
I am still waiting for my first Brain Bag, but I plan to use it with a horizontal brain cell that I already have from a Super Ego. Darcy confirms that most horizontal brain cells will comfortably fit in the Brain Bag in this thread:
3) If i got a Zephyr and a Brain Bag, could I use the same Brain Cell? Is there a way to outfit the Brain Cell with both vertical and horizontal clips?
My Brain Cell isn't due to arrive for a while (I ordered it in Olive, an out-of-stock color), so unfortunately I can't answer your other questions. Sorry!