Paul Wells looks to me to follow the right analysis about what has to happen to provoke a federal election to exactly the wrong conclusion.
Yes, it's highly unlikely that the three opposition parties will all want an election at the same time, making it a remote possibility at best that we'll see a trip to the polls caused by a non-confidence vote. And I'd add the further caveat that nearly any situation where all three opposition parties see it as favourable will likely come about only if the Cons are in free-fall in the polls - meaning that Harper would be likely to rely on his hard-won prorogation precedents to shut down any such vote before it happens.
But that means that whatever window Wells is pointing to involves a four-way game of chicken among parties who all have enough reason for optimism to see themselves having something to gain out of an election at the same time. And trying to plan election windows around that kind of rare confluence of political interests looks to me to be futile.
In contrast, given that the Cons' fixed election date legislation isn't worth the paper it's printed on, there's still an opening for the Cons themselves to call an election anytime. And it seems at least as plausible that they might prefer a fatigued public this fall to the other available windows in trying to parlay their 38% of the vote into a razor-thin majority as that they'd prefer to go this spring.
In other words, the election window is always open when the Cons want it - and probably not if they don't. Which means that the parties' current roles should probably be seen as reversed from the conventional wisdom - with the opposition mostly having the power to bait the Cons into thinking it's in their interest to call or force an election, while Harper holds the sole power to decide when that's actually the case.