Alright, so countless hours in on an open sea with no guidance, I can answer my own question 
in case it helps future drowners, perhaps making this community a little bit more ā¦ sharing?
Blueprints.
They can be called easily from Python API (which I did not realise at first) and they can do quite possibly anything you can imagine. For me, it would be more natural to write C++ directly (that WYSIWYG feel is not quite right for me) but that is another chapter I need to research.
For now, I am able to wire a Blueprint, make it accept parameters (~ what anim sequences to mix), then invoke it from within Python script to transfer portion of one animation (eye movement) to another, concatenate them, interpolate or anything.