If in the middle of the story,maybe the rest of the story would be about how Haddock and Snowy find justice and avenge Tintin's death and the rest would be about the sad resolution and the hard lessons that can be taken from this.
I really could not see that happening. Tintin would have found a way to escape his captors and capture the gang himself. I'm not sure how but that is the point: we would have to have left it to Herge.
You've compared the series to Shakespeare, but I would be more inclined to Dickens. (Someone once compared Herge to Balzac, Dickens' contemporary.) Dickens' final novel, "The Mystery of Edwin Drood", is similar to "Alph-Art" in that it was unfinished by the time of his death and leaves a lot of unanswered questions: what happened to Edwin Drood? Was he even murdered or did he survive and return in disguise in order to expose his would-be killer?
There have been many attempts, such as in film, TV and radio adaptations, to come up with a suitable ending, but I'm inclined to agree with others that nobody can guess what Dickens' imagination would have come up with had he lived to complete the story. Herge is a similar case.
I'll say though if I was Mr. Remis and I wanted to kill Tintin off and I knew I myself was dying... perhaps I would have put myself in the comic and revealed to Tintin that I was his father and try to save him but in the end fail and die with Tintin.
Sorry, but that sounds too much like a fourth-wall breach to me, which is not very "Tintin". There's his "wink-to-the-camera" on the last page of "Ottokar's Sceptre" when the Thompsons laugh at the fact that they have forgotten that they are on a seaplane, but I cannot think of any others.
One of the appeals of Tintin is that he has no family, background or friends from his pre-publications years. Other characters may get re-acquainted with old friends they knew before they met Tintin: Haddock and Chester ("Shooting Star"), Calculus and Hercules Tarragon ("Crystal Balls"); but all the friends Tintin makes are in the course of his adventures, he is not called up by a previously unknown aunt or uncle or old school friend to come round and help in some trouble they are involved in.
In my opinion, like Harry Potter in some instances, the series gets a bit darker as it progresses in different aspects. Kids who mature fast can see the darkside of the series.
Maybe, but Herge still drew the line at putting too much emphasis on death. "Flight 714" and "Picaros" for instance do have the potential for several gun battles like in a jungle war film like "Bridge on the River Kwai" but nobody gets injured or killed in the crossfire. In fact "Picaros" itself ends, at Tintin's insistence, in a bloodless coup d’état.
I think that the deaths in "Explorers" were intended because having Boris/Jorgen disarmed and returned to the hold would probably have just seemed like a repeat of how Tintin captures him after the failed attempt to launch the rocket from the moon. Wolff's sacrifice also gives it a moving touch with a guilt-ridden man trying to redeem himself for all the harm that he has caused.
Without reading "Alph-Art" again, I believe that there are a couple of deaths (maybe murders) but these are in order to build up the plot and, as has been pointed out, this was still the very early planning stage: Herge may not have included them in the final story, just focused on the art smuggling and cult worship, if at all.
I have been reading comics in which the main characters have been killed off, but it is really in order to bring an end to the series - the writer or artist being keen to end it and move on to something else. Conan Doyle killed Sherlock Holmes in "The Final Problem" because he wanted to focus on other works like historical romance. But these writers are still in the early stages of their careers (in their 30s or 40s; Conan Doyle was about 34). Herge was in his 70s when he was planning "Alph-Art". I cannot see him deciding to kill off Tintin just in order to create or focus on another series.
If Herge had intended for Tintin to die at about the same time as himself as a sort of finale then he would have known that the outrage from the fans would have been unforgivable and somehow I cannot see him doing that.