You can implement the hand rotation in the animation blue print or as physical constraints or generalized logic, and yes, you are correct, the grasping hand is an example of core features, its intended to be expanded on or referenced, not used as is without changes.
You would want to add a data layer on top of the hand to handle custom posing and limits. Its not really a plugin related concept, I try to keep the plugin to core features and leave the gameplay elements free form for the end users to not limit them any, the template content is examples of how to use the core plugin features (in the grasping hands case, the welded body driver and decomposing grip transforms to attach correctly).
As for the finger auto targeting a button, CCDIK should work fine for that (or another IK setup though CCDIK is fast and has rotation limits) and already has examples for FPS pawns, you would just want to range limit to to a zone from the fingers base pose forward vector and only ik chain the finger itself instead of an arm.