A thoroughly thoughtful post by Fortissimo deserves a thoughtful response, and this is an attempt at doing just that.
Taking few if any liberties with the content, the main point (or certainly one of the main points) is "a full Annex 13 investigation is neither required nor necessary" because, unlike MH 17 and Pan Am 103 where "the causal event for both those needed to be established", in the current case the causal event already is known. While I agree with nearly all the points made, still my contention is that Annex 13 should be "worked" - for reasons other than establishing the causal event.
And yes, even though the standard orthodoxy in public international air law is that Annex 13 was not written for this purpose. In a phrase, both that orthodoxy and the reasoning in Fortissimo's post prove too much (as lawyers are apt to say) - their position does not deal with all possible cases or reasons.
First, some items from the news, both for context and support. Jan. 13, CBC News, "Canada investigates reports that Iran is harassing families trying to repatriate remains of crash victims". Article on CBC website (I don't think I can post links) reported Ukraine's ambassador to Canada had informed Canadian officials that repatriation could take days or weeks, or even months. The article also reported that TSB had sent a team to Iran as well as Global Affairs CA sending its Standing Rapid Deployment Team. And that a group had been formed, led by Canada, the International Coordination and Response Group, including Ukraine, Sweden, Afghanistan and the U.K.- "to ensure transparency and accountability in the wake of the crash."
And, Jan. 14, on the "north shore news" site, also id'd as The Canada Press, "Zelensky says he, Trudeau talk crash punishment, as Iran announces arrests." The article, in reporting the content of a phone call between the two national leaders, states that Canada's Trudeau said the call had "underscored the need for a full, complete and credible investigation with international participation" to provide "answers, accountability, and justice that all those affected by this tragedy deserve." Not least the report indicated the Int'l Coordination and Response Group is meeting in London, Thursday.
For three reasons, Annex 13 continues to hold a central place in the aftermath. First, as a guiding principle, or set of practical guidelines, for proper handling of an accident site and the remains of crash victims, Annex 13 certainly has been until this incident, the unquestioned coin of the realm, gold standard and highest authority. What Iranian authorities may have done or not done obviously is over with now, but following Annex 13 as much as possible would serve to uphold its primacy. And it might even lay down some guardrails for what ongoing activities are yet to occur . . . a comment reliably attributed to a knowledgeable official about repatriation possibly taking more than weeks is some significant cause for concern.
Second, and the aviation expertise I lack may be flagrantly showing on this point, but is it not the case that having the kind of in-depth, technically high-virtuosity inquiry into the operational details of the flight would still be very useful and very important, even though the cause of the loss of the aircraft - a SAM firing or more than one - is known? As an example, did the ADS-B cut out as a result of a missile detonation, or because of something else? And more broadly, isn't everything on the CVR and FDR relevant to having a complete picture of what happened, again even though the missile or missiles as the cause (in an immediate sense) are known already?
As the Iranian authorities report on their findings, such as they might turn out to be, having a complete operational record from the nominal investigation process certainly would seem a wise bulwark against possible shading of the truth.
Stated another way, why not push the Annex 13 process as much as feasible and possible? Nothing I have read indicates that doing so would interfere or in any other way be a detriment to Iran's inquiry or inquiries (in its military, by its courts and criminal justice authorities, and otherwise).
And third, the Annex 13 process could be the single best way to support the nascent International Coordination and Response Group in its efforts, though there is little reporting so far on precisely what its portfolio will be. That is, to the extent Annex 13 investigation and inquiry is not "necessary", certainly there is no contention that the rest of the world in the global international civil aviation sector should just sit back and wait for Iran to tell us what it decides to tell us. And so the nominal process under the Chicago Convention could very well serve as the prep cook for the big-name chefs of the Int'l Coord. & Resp. Group.
There is one other subpart, of course: keeping this from happening all over again, or trying.
Cynicism about the pronouncements and promulgations and papers of regulators having any significant effect in the operational world runs deep, but it is both impossible and pointless to disagree that leaving the decisions in the hands of commercial interests will lead to predictable recurrence of bad results. I happen to believe that deploying the Annex 13 process as far as it can go in this case will yield benefits when the actually smart people on this issue all get together next, and even if that turns out to be incorrect, I can't see that it could hurt. Alert statuses for air defense units around the world aren't likely to go away any time soon, and it seems more than just plausible that anything that could provide some leverage on the problem should be endorsed, and given a try.