IT2013: I am certain that the mystery game is on this list, because it has to be one of these games, according to the clues:
Airborne Assault: Red Devils over Arnhem,
Blade of Darkness (otherwise known as Severance: Blade of Darkness or Severence: Blade of Darkness),
Football Manager 2010,
Meridian: New World,
NBA Live 2004,
Operation Flashpoint: Cold War Crisis,
Operation Flashpoint: Resistance,
Pitfall: The Lost Expedition,
Return to Castle Wolfenstein: Enemy Territory,
Rune: Halls of Valhalla,
Snoopy vs. The Red Baron,
Space Rangers 2: Dominators (the original, "vanilla" version),
Victoria II: Heart of Darkness,
Wanted: A Wild Western Adventure
Of all the PC games that have user ratings of 9.0 or higher ("universal acclaim") on Metacritic, all of the games that are not listed above are disqualified from being the mystery game, because they are either on Steam, or on GOG, or have 100+ wishlist votes on GOG, so that would contradict the clues. Or they have been previously guessed in this thread, in which case RadonGOG would have said, "You got it right" to whoever got it right.
No, it hasn´t been already guessed...