So, here's the diagnosis :
I messed up with wrong parameter references to the multiparm block list. Most of the prim() and point() functions were trying to read attributes that didn't exist [embarrassed].
I revised almost all variable names. It's neater and working now.
Also revised the PythonModule functions. They won't throw now if the attribute name string filed is cleared (by hand).
Also took care of the attribute drop down menu scipts. Vector attribs won't be listed if the Attribute to Operate on is a float. That's true also for the lerp operation attrib menu. (Don't ever try a vector2 or vector4 by manually typing its name. It probably won't go well.)
Finally, a description parm with a python script gives you the input/output attrib names at a glance.
Just for comparison, a demo of v2.0 (the previous version) here, which was before vector3 support. It makes it a bit clearer what I mean by attrib blending (scrambling):