KeyMath is a good test as it slices and dices different math skills very finely, much more closely than WJ. I think there are 10 subtest scores that should get at everything from numeration to probability to geometry. It's also untimed, which should be an advantage to getting you valid information. You need the scores on each section, as well as observations. Based on what you describe, I bet you'd find significantly different levels of performance across the different domains, even if she's given an "no problems" overall level of performance.
If you suspect she hasn't mastered Piagetian principles, a good place to start is the math developmental stages on the PBS website:
http://www.pbs.org/parents/education/math/milestones/preschool-kindergarten/as these are things that kids generally master as preparation for learning math in school. Identifying holes there will be useful.
How to proceed? In your shoes, I'd find her strengths (KeyMath should help some) and teach via those strength, adapting and accommodating as much as possible.
I'm curious - she strikes me as a kid who would thrive with formal logic. It's math with no numbers.
