Try this to correct the fingers.

Apply a T-pose to your character.
Click the "Edit Mesh" in the Modify panel.
Click the Face tab.
Check the box for soft selection.
Click right on the end of the finger.
Adjust the soft selection so it only effects the finger you selected.
Select the rotate gizmo.
Roll it until the finger is oriented how you like.

4u2ges (7/17/2021)

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[quote]Meantime you can have this fixed one as a sample

OMW, that's amazing, thank you!

The hand replacement strategy looks really promising. I'll give that tutorial a go. Thank you so much.

4u2ges (7/17/2021)

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[quote]Apparently, it's not a problem with rolled bones (at least with the one you have attached). They are all straight in Blender.

I don't know about that, in blender I see a difference. The default character's xaxis points directly away from the nail, my messed up hands don't!

Yes, sorry, I should have mentioned.
The Bind pose character came with is all wrong - it is not only fingers. At that point there was nothing could have been done to fix it, but to restore a correct T-pose (https://forum.reallusion.com/FindPost469889.aspx ).
Which is what I did first. Once that done, bones are straight, but mesh remained deformed to some degree. You know the rest

Thanks again for all the help @4u2ges and @Rampa

I created a blender script to transfer the roll, based on a vertex normal for each bone. It finds the closest vertex normal to the zaxis of the bone, then works out a fudge angle on top of that. It's working fine for my needs.

Don't judge my python https://gist.github.com/dantreble/24c46ca5f7eb61f8e51f08d6b6cf2322 !

That's cool Dan. Might come handy one day. Thanks...