You probably don’t need to fiddle with the keys and device ID:s actually… Just stop syncthing, copy over the index.v0.11.0.db directory, and start again. You must make sure that the contents of the folders on both sides are identical at this point, since that is what you are claiming by copying the index. Any missing files will be seen as deletes, and so on.
I haven’t actually tested this myself. Good luck!