[PK]

It's an interesting take, but I think that it's a mistake to change FAE from 4dF to 3dF for the sole purpose of trying to match the canonical Intervention rules. If you want to use FAE, then use FAE. Don't try dragging mechanics over from one system to the other.

Here's my suggestion: Whenever the dice come up ++++ or ----, roll one extra dF. If you end up with +++++, it's a a Divine Intervention; no FP can be spent to reroll or modifier it! A ----- is similarly an Infernal Intervention. Otherwise, just ignore that fifth die and treat the roll as normal.

The odds remain about the same. The d666 have a 0.46% chance of producing a specific Intervention, while 5dF have a 0.41% chance of coming up all + or all -. And IMO, this is just as flavorful, since it's basically saying, "If you roll (the equivalent of) a critical success or failure, see if powerful forces take it one step beyond!"