From my experience GMing CoCT, Zellara was a great element and one that the party enjoyed. In Scarwall when
Spoiler:
Zellara's spirit was absorbed by the castle the party reacted with grave concern wishing to rescue their friend.
Additionally, she is helpful as a way to give the party some direction if at a loss for their next step.
If you do remove her then I would think you're jettisoning the harrow points (which is fine they are not vital in my opinion) but shouldn't affect the campaign greatly. You may have to think of some alternative for the last adventure
Spoiler:
for the deck of many things in CoF
My group just started chapter 4 and they enjoy zellara so much they named their group Zellara's Revenge.
That said, you know your players so feel free to customize you your hearts content.