Is it possible to rig to the UE4 skeleton if finger positions are in a different position?

I have hit a wall on my project. I am working on some first-person arms for my game. The fingers are modeled in a different position to the UE4 mannequin. I was able to adjust the rig’s proportions using the ART tool and change the rotations to match and skin my fingers.

All the rotations are facing the correct direction/orientation, the skin works great and I’ve been able to polish it to a point where there are no obvious issues.

This is where I hit a wall.

My understanding is that to retarget an animation the joints have to have identical rotation, but the positioning can change for different proportions.

So right at the end before exporting to UE4, I set the rotation on all of the bones on my adjusted skeleton to 0 to match the default skeleton.
When I do this, it ruins the topology, and the fingers look very strange.

If I leave the rotations at the custom values, the skin is great and the fingers move properly, but then retargeting doesn’t work. I feel stuck at this point and no tutorials cover this.

I attached 4 screenshots, two before, and two after. Any suggestions would be hugely appreciated. Thanks heaps in advance.