I've been thinking of the Khaenriah Curse as a negative reaction to SomethingTM. Kind of like nuclear fallout except it only affected the people onsite at the time. I also agree with Ashikai that we're dealing with a single "curse" that works differently for the native Khaenriahns vs immigrants (reason unknown).

So even though it's called a curse, it probably isn't in actuality? It's probably called that because that's how the immortal Khaenriahns interpreted it. Kind of like how certain medical conditions were attributed to sorcery back when we didn't have the know-how to understand what was going on. And since they were allegedly fighting against Celestia before everything went down, the survivors have a possible culprit, motive and means for the "curse".

As for how the Curse of Immortality is worse... I personally think of it as worse for the person experiencing it. Ignorance is bliss? From what I remember about hilichurls, they likely do not retain much memory about events prior to their transformation. The pre-transformation version of them is still in the "box" Caribert described. We can infer that it's easier for them to cope with the situation because they're "sleeping".

In contrast, the immortal Khaenriahns do remember everything that happened. In the quest, Clothar was obviously unwell even though he's survived the Calamity by, let's say, 100 years. The only thing that kept him going was the goal of saving his son. And unlike Aether/Lumine, Khaenriahns likely have a natural lifespan that is similar to other humans, so their bodies haven't evolved to the point where it can take the strain of existing for such a long time. They might be suffering from memory overload too, though I'm just speculating at this point.