Let's answer Greg Lyle's headline question as simply and concisely as possible:
The NDP's opportunity in the ongoing federal campaign has never involved the ability to move the election date up to fit a rise in the polls, nor a plausible expectation that well-funded opponents would let that rise go unchallenged. As a result, we shouldn't judge any campaign by the fact that the election remains a three-party race.
In fact, it would be a waste of resources to focus unduly on pressing an immediate advantage which would likely be undermined by election day. And that has to be considered a real danger for anybody who rises far enough to become the main target for all of the other competing parties.
It's true that so far, only the NDP has managed to rise far enough above the competition to reach front-runner status for even a moment. But it's hardly a negative for the NDP that it has already shown it can do so. And the fact that it managed the feat while engaged in significantly less advertising than its competitors should hint at the remaining room to grow once the NDP makes its push toward election day.
Ultimately, the campaign can only be judged by where a party ends up. And while the NDP should absolutely be adapting as the election approaches, its opportunity to form government is as strong now as it's ever been.
[Edit: fixed wording.]