I can't get the rest pose to reset without the rig glitching

So I’m trying to rig a sword to the hand of my character so I parented it to its on bone on the rig. Now I’m trying to reset the rest post of the rig before I Parent the sword bone to the one in the hand.

Here is the rig before I do anything.

Here it is after I duplicate the character’s Armature modifier. See the fingers go all over the place.

In this one I applied the new rest pose, which it does in edit mode but in object the hands have moved together and resetting their transform does nothing. Plus the fingers have glitched more.

In the last one I applied the first Armature modifier and nothing has changed with the pose of the mesh itself. I could really use some help here, I worked really hard on this mesh and I don’t want to start over.
(Edit) Here is the blend file

Use one of the file-hosting sites…Mediafire > Pastebin .Google Drive, then send us the link…
Make sure that the file is sharable on whatever site you choose.

Thanks, I added the link to the file to the first post. Any help I can get would be greatly appreciated.

So I decided to trying changing the rest pose on another rig on a separate file that I had changed the rest pose on before on an older version of blender. The same thing happened on that rig, so clearly its not particular to that first rig, it must be something I’m doing wrong when trying to change the rest pose, now that I am using a newer build of Blender. I guess I don’t know how to do it in 2.91.
So can someone walk me through how to, and avoid this problem?