Here is the full list of all 10 Only If achievements. It takes around 2-3 hours to unlock all of the achievements on Windows.
Complete the White Pawn Scenario.
Complete the last level with a single bullet.
A sizeable achievement.
Find the developer's future game concept art.
Solve Vinny's riddle on your first attempt.
Complete the Black Pawn Scenario.
Welp, there you go.
Screw around in the fireplace.
Press E on every single bloody thing.
Stop looking at the damn picture.