Imran does believe that the outcome of the election would've been different if "fair" elections took place. The thing is, I have yet to see an independent analyst/group that agrees with his contention (most agree that election rigging did take place, but not to the point that it would've changed the outcome of the entire election). In addition, not only does Imran think that election was rigged, he also believes that a vast group of people acted against him to ensure that Nawaz would win. This includes the Supreme Court and the biggest TV station in Pakistan, GEO TV. Once again, no independent person/entity agrees with this assessment.Electoral reform is always a bit of a catch-22 issue though. If a poor or unbalanced electoral system is preventing you from winning elections you believe you otherwise would, then you need to gain power to change it.. but in order to gain power you need to win elections.. which means you must first enact electoral reform.. which means you must first win elections.. and so on.
Does Imran believe (or claim to believe) that the ultimate result of the election would have been different if the electoral system in Pakistan had been just? If so, whether violence is justified actually becomes a murky issue as it's possible he believes he's being tyrannically oppressed by an illegitimate government, in which case he'd be acting in self-defence. Sure, he's probably just making a power grab, but it's a bit more complicated than "if he wants to enact electoral reform, he must do so by first winning a rigged election", surely..
Given that Imran has no concrete proof of his allegations, nor the backing of independent commissions to support his assertion, I feel he has no moral ground to try toppling an elected government via street power. I also am astonished that he fails to see the long term implications of his actions. Suppose that he succeeds, Nawaz Sharif resigns, and fresh elections take place in which he wins the majority of seats. He has now established a blueprint on how to take down a government. A year or two from now, some other revolutionary with his list of complaints will do the same to Imran's government! Democracy will never prevail in Pakistan if violent protests are all that are required to take down the government.
The correct course of action was for Imran to build support for his views with the other opposition parties. Together, they would've easily pressured Nawaz Sharif to agree to electoral reforms. Imran could also use the constitutional method of dismissing a sitting PM via a no-confidence vote in the parliament (again that would have required him to build support for his action with the other opposition parties). IMO, there were many acceptable course of actions that Imran could have followed, yet he choose to follow the most destructive and dangerous one.