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 Black Pawn Scenario.
A sizeable achievement.
Complete the White Pawn Scenario.
Complete the last level with a single bullet.
Welp, there you go.
Solve Vinny's riddle on your first attempt.
Find the developer's future game concept art.
Screw around in the fireplace.
Stop looking at the damn picture.
Press E on every single bloody thing.