They can't call Farrell up and send him back. Once he joins Montreal, he can't go back to the NCAA. The expectation is they'll try to sign him this season but they'll have to wait until after his season ends.
They could call up a signed CHL player (Mesar, Beck, Roy, and Kidney among forwards) but is this really a good situation to throw them into? Two of them were just traded and are settling in with their new teams; is it worth disrupting that transition to toss one of them onto the fourth line for a week and a half before the All-Star break? Probably not. Mesar isn't remotely ready; maybe Roy could handle himself but it'd still carry some risks.
The Habs don't need an emergency situation to recall Heineman. If they wanted to, they can bring him up at any time (cap and roster space permitting, of course). But again, now you're flying someone over from Sweden for what could amount to a four-game stint and then sending him back, disrupting his season.