2.
3. Should work just out of box.
Does not required any game progress to work.
4.
5. Does not affecting your save data.
6. Does nothing else, only OG Papa nier as main playable hero.
The creator of the NieR Replicant Ver.1.22474487139... Papa Nier mod also provided some details on known issues, which you can find below.
Known issues:
1. In some very rare cases (only on the on first load after game launches) you may be loaded as brother Nier. Just trigger loading zone or reload save.
2. Might not work when launching game directly by running "NieR Replicant ver.1.22474487139.exe". Recommended to run game through Steam.