Why didn't Plutarch Heavensbee tell the tributes with whom he was already in communication more about the arena for the Quarter Quell? For example, none of them seemed to know that the arena was a clock. They don't seem to be pretending. For example, Blight (male from District 7) died by running into the force field during a rain of blood. And it's even stranger that he gave Katniss the enigmatic hint "it starts at midnight," a hint that she had little hope of putting to use, if he wasn't going give useful information from people who understood the source was on their side.
He needed things to happen in a certain way to keep his cover and if people survived then that would probably not be possible. Odds are the "higher ups" would have put 2 and 2 together to find out what was really going on and that would have endangered everyone.
After the end of the Games, Seneca Crane is executed, and Plutarch is appointed to be the new Head Gamemaker, citing that there were not many takers for the job.
It wasn't a war he could fight. He was a survivalist he survived by any means.
As such he was an ingenious strategist but this also caused him to be somewhat callous as he showed no concern for people who died in the Quarter Quell or the war.
He needed a realistic chain of events to occur, some die, some get killed and some survive.This way he has a good life and life security if everything goes the way it is suppose to.
I can't find any canonical evidence, but the most logical answer would be that he was trying to avoid the same fate as Seneca Crane while still helping as many tributes as possible.
Crane, as you probably know, let Peeta and Katniss both win the 74th Hunger Games instead of letting them commit a double-suicide. There was probably a way to have crowned one winner, like using the arena against one person in some way, like when they started the forest fire on Katniss to turn her toward the other tributes, but instead he just let them both win. As a consequence, he was executed.
Heavensbee was obviously on Katniss's side, but he couldn't risk the Capitol finding out about that or else he'd also be executed. He tries to give as many subtle hints about the arena as possible, like showing his Mockingjay watch to Katniss during the Victory Tour. Unfortunately he can't be any more forward about it because the Capitol might figure it out if the tributes know their way around a little too well.
Haymitch tells Katniss that they couldn't risk telling her very much because she could have slipped up while talking to someone and they would've gotten caught.