I've always treated non-Fae Scions almost exactly the same as Changelings - you have to make a Choice at some point, spending more refresh on powers brings you closer to the Choice, etc. It seems the simplest, most reasonable way to handle these things.
The Choice will have to be made at some point... What's that point? Can it have a mechanical, game definition?
For example, in the
Neutral Grounds play scenario, there is a Half-Elf named Dania. Her power is
Glamours [-2]. In her write-up, they go on to say that she has the ability to develop more Fae powers such as
Inhuman Speed and
Inhuman Toughness. If she develops these powers, she will have given herself over wholly to her Fae side and become an Elf, turning into an NPC at the end of the gaming session. They are playing in a Waist Deep scenario. She only has 7 Refresh to begin with and when you look at all her Stunts and Powers, taking those two aforementioned abilities will take her Refresh to Zero. That's the point where she loses Free-Will and becomes an NPC according to the book.
But lets say this character stuck around for a longer Chronicle and achieved a Major Milestone, gaining +1 Refresh. Now, even if she develops
Inhuman Speed [-2] and
Inhuman Toughness [-1], she will still have 1 Refresh and still, technically, be a PC. Has Dania staved off the Choice for a little while longer? Can she keep it up if she takes things slowly?
Or, is it inevitable? No amount of Milestones or Refresh is going to prevent her from becoming wholly Fae as long as she keeps developing supernatural powers? Is that meant to be a conceit of the game? You can be a Pure Mortal or a Spellcaster and remain human indefinitely as long as you make the right choices. Anything else... even when you make the right choices...you're doomed to become a Monster.