Nick,
It was always the intention to support flex arrays in the absence of Numpy. If there is some refactoring to be done, this principle should be preserved: that Numpy is an optional rather than required dependency.
Not sure about the wording in your second sentence. At the time we developed flex, branching was not a code development mechanism we used. Furthermore, not sure why you say Numpy is "exclusively" used in the flex constructors? Certainly there are numerous flex constructors that do not involve Numpy?
Nick Sauter