Using those recently-released level notes, I have deduced that the authors of the Marathon 1 single-player levels are
Arrival -- Greg
Bigger Guns Nearby -- Jason
Never Burn Money -- Greg
Defend THIS! -- Alex
Couch Fishing -- Jason
The Rose -- Greg
Smells Like Napalm, Tastes Like Chicken -- Jason
Cool Fusion -- Jason
G4 Sunbathing -- Greg
Blaspheme Quarantine -- Greg
Bob-B-Q -- Jason
Shake Before Using... -- Alex
Fire! Fire! Fire! Fire! Fire! Fire! -- Jason
Colony Ship for Sale, Cheap -- Jason
Habe Quiddam -- Jason
Neither High nor Low -- Greg
Pfhor Your Eyes Only... -- ?
No Artificial Colors -- ?
Unpfhorgiven -- Reg/Greg
Two Times Two Equals... -- ?
Beware of Low-Flying Defense Drones -- ?
Eupfhoria -- ?
Pfhoraphobia -- ?
Ain't Got Time Pfhor This -- ?
Welcome to the Revolution -- Jason
Try Again -- ?
Ingue Ferroque -- Jason
Checking on http://marathon.bungie.org/story/levelcredits.html reveals that the only corrections are to "Arrival" (Greg and not Jason) and Reginald DuJour being the coauthor of "Unpfhorgiven" instead of "Pfhor Your Eyes Only".