Choose which character to load?
I've found that switching save games between my Windows and WinMo phone, the savegame name is different.
I would recommend making a dummy game on your target platform and seeing what the expected file name is, and then copy your real file and add the extra information.
For instance, on Windows, the filename is <win-username>-<Character-name>.Nethack-saved-game, as you say.
On my mobile version, however, the filename is nhsave-<Character-name>.Nethack-saved-game.