Why not use vegan cream cheese? If you add enough powdered sugar to it, it will stay firm.
And if she can eat it, a good quality white chocolate works really well and lets you reduce the amount of uber-sweet powdered sugar. Of course, you would have to use a vegan bar there too.
Another option is to only ice the middle layers: split the cake and have three iced layers, then pipe your personalization directly on the top surface (use a bottom layer for the top) with chocolate. That way you don't have to worry about a crumb coat and the visible alternating colored layers will be more vibrant.