I'm not reading this whole thread for details on everyone's questions/arguments, but to try to give a broad answer to the thread title that doesn't spoil anything:

First, the main difference between Durkon's situation and other vampires is the speed at which everything happened. A standard vampire gets three days in the grave to absorb the lion's share of memories, and then takes months to slowly assimilate the rest. This vampire didn't get any of that, and so they were overwhelmed. A vampire who absorbs the memories in the "proper" way will not be overwhelmed. It could easily be argued that this would be a "balancing factor" in Malack's swift-rise spell, if I was going to stat it up for actual D&D play (which I'm not). And that's an aspect that Hel and her newly-created minion wouldn't necessarily know about, because it was Nergal's spell.

Second, unlike the other vampires, he needed to access those memories right away because he needed to impersonate Durkon. The Exarch is not going to be in as much danger of something like this because nobody cares what spirit is in charge. He can (in theory) just put off even looking at all but the most basic of Gontor's memories until this whole thing is over, and then sip them slowly over years.

Third, I doubt everyone has a single memory that could surprise and shock the vampire spirit like that. Because it's not the speech about "worst days" that does the real work here; that's mostly just Durkon psyching himself up by telling the spirit that they're wrong. No, the workhorse is the memory itself and how it makes the spirit feel. And I just don't think most people have a single ten-minute memory that completely changes the context of their entire life, before and after.

And finally, this would be the sort of thing that would take a very strong will to pull off successfully. Even if you explained this entire procedure in advance, I doubt someone like Haley or Elan could make it work. They just don't have the mental strength. They could show the spirit the memories, but without the unshakable resolve to back it up, the spirit wouldn't be as affected by the emotional content. In purely game terms (which I usually don't like to discuss but this is good to use as an analogy), Durkon is a high-Wisdom, high-Will-save character who possesses an unimpeachable Lawful Good alignment. That makes him unique. He may, in fact, be literally the single strongest willpower character who has ever been vampirized, ever. It's not a thing that usually happens to Lawful Good clerics in their mid-teen levels; Malack would have been much lower level (and not LG) when it happened to him.

The best way I can put this, overall, is that a human needs about half a gallon of water a day, or about 93 gallons over the course of 6 months. But if I poured 93 gallons of water down your throat today, you would die. That is not a significant design flaw in the human body that needs to be addressed! It's just a circumstance that doesn't come up enough for us to spend all of our time worrying about whether or not we are in imminent danger of swallowing 93 gallons. And if someone offered you a drink, you wouldn't think about, "But what if they suddenly whip out a fire hose?" before accepting.

So, is it a thing that could have happened to other vampires? Sure, maybe, once or twice, just as I am sure people have died from drinking 93 gallons of water. Is it common enough for Hel to mention it in her five-minute orientation of the vampire spirit before stuffing it in Durkon's corpse, when she has an entire scheme to explain as well? No. At best, she would have said something like, "Keep an eye on the host spirit, don't let it get control," and the vampire would have said OK. And then still walked into Durkon's trap because it was incapable of connecting the dots on its own beforehand.

(Also, fair warning: I'm not going to get into a back-and-forth on any of these points. If you still don't want to accept that this is a pretty rare set of circumstances, I don't know what else to tell you.)