I agree with ZealuxMyr about trigger and resolve timing: they are not necessarily the same, they can be different from each other.
Most of our analysis about this case is based on interpretations about Harpooned been resolved on step 6, 7 or between, making this the first card, that I can remeber, that could be able to interrupt an attack effect and resolving it's own effect before it, creating another type of interaction in the game, I don't like this approach but it's a possibility, and only a FAQ will be able to solve this without a doubt.
What really makes me unconfortable, is that a lot of people that I know is using this way of thinking to do an Harpooned TLT exploit, just because they can, and ignoring a lot of others signs on rules and other card behaviors clarified or changed on the FAQ. It feels like: "Ok, I can see a lot of arguments and other mechanics on the game that indicates a conclusion where TLT effect must be resolved before another effect of other card, but because FFG made this miscalculation on the wording I can use it the other way around, who cares about how the game designers was thinking when they created this card, let's explore it like we can, while we can". Obviously not all players defending TLT + Harpooned as a valid sinergy think like this, but there are a lot of examples, that will not be taken on consideration by a lot of people because the wording is not exactly the same, showing that probably that's not how we should interpret TLT attacks and effect, the steps and the term "uncanceled"(hit symbol) and "uncanceled"(critical symbol), but people around my gaming circle just discard any argument because:"the wording is different, unconclusive but different", like it's never happened before with other cards.
Ok, the wording of Harpooned is different and give us a margin of interpretation and error, I get it. Hear me out, there's an effect that clearly is triggered and resolved on step 6, even before the attack hits, that take in consideration that canceled critical result dealt by ion canon (and any other kind of weapon that cancel it's own results) is not considered valid. I am talking about Imperial Kath Scarlet, and is one argument and example that shows us how these kind of weapons work, they don't generate damage or critical damage like other weapons, but people refuse to talk about it because harpooned is not Kath, that's a rule only for Kath, the timing is different, the exactly choice of words is different... etc, etc, etc... Well, if it's something inconclusive, why not rely on the examples of the past, even if they are slightly different? No, that's wrong, let's use the exploit. The exploit is good. The exploit is a better solution, let's try to figure it out a cranky ruling about initiative and who "owns" the card, and even XX-23 S-Thread Tracers, a card that dispense FAQ because any other precedent examples of other weapons, will be able to activate Harpooned, a missile with 4 squad points that don't spend TL to be used. That's how I am feeling right now, no more #flycasual, because the people around you will behave like if they are in a competitive cenario of a mainstream card game competiton. Again, not everyone that defends this sinergy think like this, but there are trolls everywhere that will not use arguments and logic.
I beg your pardon for venting this out. I am sorry by my outburst. In short, like ZealuxMyr explained before, and it's something really important, triggering and resolving an effect can occur on different timing. Why Harpooned can't be triggered when hit but resolved on step 7, this is unthinkable? Why necessarily Harpooned condition need to be resolved imediatly when triggered? Why we are interrupting the effect of the TLT attack, to solve another new effect that triggered on the same time that an effect of the attack in motion? Why we are ignoring other possibilities with more background just to exploit TLT + Harpooned?
ps.: This right here is only my personal speculation: I think that we are looking wrongly to the wording "canceled/uncanceled", it feels like "Schrodinger damage" until step 7 when we can finnaly open the box.