The overlord card "Reinforce" (Warlord III) states: "Play this card at the start of your turn and choose one of your monster groups in this quest (except for a lieutenant). You may place monster figures on the map up to the group limit for that monster. These monsters may not be placed within 3 spaces of any hero, but may otherwise be placed in any empty space."
Does this card brake the game?
I haven't heard any accounts of this card breaking the game. You raise some interesting applications, but I'm not sure they're entirely unstoppable.
In quests where the overlord has to occupy designated spaces, he can just place these monsters on those spaces for an instant win.
In quests were doors have to be unlocked in order to reach your goal, the overlord can just place monsters directly into this room for an instant win.
Usually, in quests that require monsters to occupy specific positions, it's a specific LT who must occupy that position (and this card doesn't work for LTs.)
I can see how it could be used for a quick point or two in First Blood (reinforcing goblins onto the Exit tile) but that sort of tactic can be prevented by the heroes not slaughtering all the goblins on the map at once. (The card still requires the OL to respect group limits, so if only one goblin has been killed, only one goblin can be reinforced.) Unlike 1st edition, there is no rule (that I'm aware of) which says the OL can voluntarily remove monsters from the map in order to reinforce them elsewhere. Monsters also aren't allowed to attack other monsters in 2E, so the only ways to get monsters off the map are to have them killed by the heroes or to move them off a quest-designated exit tile.
Also, at least in some quests, the heroes can try to position someone within 3 squares of the OL's target position in order to prevent him from reinforcing there. Remember too that many quest NPCs (like Sir Palamon, for example) are counted as hero figures, so monsters can't be reinforced directly beside them.
I'm not saying your hypothesis is wrong, mind you. I haven't played enough to say for sure yet. However, there are ways the heroes can control the potential risk of such things by not killing certain monsters or by keeping guard when possible. As long as everyone is aware of the play and plans accordingly, I think it's okay.
Can monster groups which are not on the board (yet, because they are in reserves and are placed on the board at a preset condition), be placed prematurely.
I'm inclined to say "yes." I might be convinced to house rule it otherwise if there's a particular example of a quest where this really messes up the balance, though.
Official reply from FFG on the way.
I'll be interested to hear their answer. As I said before though, I don't think it's a big deal either way.