I doubt Raiden or any of the Earthrealm warriors could do much to save Sub Zero given the fact that they were in the middle of a colosseum with thousands of spectators watching, almost all of Shao Kahn's minions being present or ready to on call, a whole bunch of the Lin Kuei, Sektor, Cyrax and most importantly Shao Kahn himself being present, there was really nothing much they could have done.
Raiden could have EASILY saved Sub-Zero the same way that he saved Smoke in the Living Forest against 4 robots, Shang Tsung, Reptile and Kano.
Afterwards, he could have teleported himself and the other 4 the same way that he teleported himself, Liu Kang's corpse, Johnny Cage and Sony Blade after he defeated Shao Kahn.
There's always an explanation and a possible solution for it.
The fact that ED Boon did it anyways because "a character that wasn't cyborg may be cyborg this time around," is another story.
As for Shao Kahn's minions, the only people I remember being in the arena were Kintaro, Ermac, Shang Tsung, Quan Chi and Reptile.
Kitana had left the arena because Kahn had been mad at her for allowing Cyrax and Sektor into the arena.
Scoprion is neutral.
Raiden, Kung Lao, Liu Kang, Johnny Cage, Smoke and Sub-Zero were also in Kahn's Coliseum and I don't see how they couldn't have evened up the fight.
Plus, what does spectators have to do with it?