@JakeMorph, 2 things.

#1 I agree with you about the Jack-universe thing on page 1, I forgot to mention that lol.

#2 I never actually thought to connect conditional (im)mortality to the "durability" of the characters, though now that you mention this I do think this goes partway to explaining the discrepancy. I don't think it covers it all the way, but I do think it's helpful.

Namely, what form do these mortalities take? God tiers can clearly "die" from things that aren't narratively important (that's where the middle-of-the-road judgements come in), so although they might be fated to die in important circumstances, I'm not sure if that changes their physical toughness in any way. In general this might counter-intuitively imply that less important characters are significantly tougher, since there's no important way for them to die lying around.

Perhaps more relevant would just be the Condesce herself. When Vriska is planning around her in 7520, she doesn't seem to have anything to say about conditional mortality. Just to "keep hammering away at her until she's dead". If she knows about the mortality, this implies she thinks it could be circumvented by just "trying harder". If she doesn't, this raises another question. Did HIC die because her immortality was removed, or was her immortality removed because this is when she was fated to die?

Rewording, is it that English just happened to remove her immortality moments before the fight started, or was her immortality just that English set up the timeline such that she wouldn't die until X moment? Is Thor "immortal until Ragnarok" because he's tough or because he's going to die at Ragnarok and not anytime before then?

Beyond this, I also feel like Homestuck tends to avoid things happening "just because the plot demands as such". Just and Heroic deaths are clearly based on narrative, they basically don't even exist outside that framework (define "hero", lol). At the same time, the deaths are still based on actions in that narrative. However Cascade-y the events around them might be, a god tier won't die if you just shot them at random. They'd still have to like, be doing something. In this case, what's the "doing" that makes you "weak to knives and guns"?

Maybe it's bimodal? If you're at the threshold of a Just/Heroic death, then you can be killed by the blade. But you do have to be doing *gestures* "things" that would justify that in the first place. If Jack just randomly swung his sword at Dave, it'd just "decrease his vitality gel" or whatever. It's gotta be at the end of a fight, and it's gotta be cool (both conditions that'll likely mean his "health" is gone regardless). Maybe. Perchance.