Free Will

The second achievement can be unlocked, if you unlock it in offline mode and have it synced to the steam servers later. This will seemingly prevent the other achievement from relocking. It requires a second device to unlock and cache the other achievement offline.
I have tested this on a steam deck, PC would probably work, too
Step by step, here's what I did:

1. Play the game on PC until i'm in Wendy's room
2. Make a backup of the savegame.
3. Install the game on my steam deck, copy save from PC.
4. Take Steam Deck offline (offline mode, disable wifi)
5. Finish the game on steam deck for the "A Letter from beyond" ending.
6. Advance Steam Deck clock by 72h
7. Relaunch game, unlock "A Letter from beyond", exit game
8. Back on PC, finish the game for the "Our appologies!" ending/achievement
9. Take Steam Deck back online, "A Letter from beyond" should sync to the steam servers after a few minutes.