For modifying the edge snap distances, you'll need to open the character's .pac file in a hex editor and go to:
[[[[0x64] + ([0x68] * 0x4) + 0x80] + 0x90] + 0x94] + 0x80
Square brackets represent the value inside the address indicated.
From there the values are:
+0x4 Minimum distance below edge before edge snap.
+0x8 Distance window away from edge.
+0xC Distance window after minimum distance below edge.
The 0x8 value allows you to modify the distance, but it affects both forward and back edge snapping. You can use the global multiplier for backwards edge snapping to adjust that, but it will apply to all characters. (likely courtesy of the fighter.pac file)
Code:
Reverse Edge Snap Modifier: [Phantom Wings]
4A000000 80B883A8
14000000 XXXXXXXX
Where XX is a floating point value.
Just remember that it only modifies the horizontal distance, so if you manage to be exactly under the edge, you will still snap to it.