Just some ideas for new Game options people could select to make the Party more durable. I REALLY hate reloading all the time, because eventually it just seems like luck to beat an encounter, although I do enjoy difficult encounters, so these suggestions are fairly straightforward changes that can improve party durability without necessarily requiring the module to be changed.
Apply an automatic"short rest" after each encounter, where characters recover a percentage of HP and ability/spell uses. The percentage could be set independently for HP, spell slots, and ability uses, so you could opt for 15% HP recovery, 40% spell slots and 50% ability uses, for example. It could range from 0% (by the book 3.5e) to 100% (full recovery after each encounter, more along the lines of Pillars of Eternity 2). In this way, people could adjust difficulty to their liking, depending on the module difficulty, and how often they want to reload the game due to character deaths. This should be fairly easy to implement, since you already have items and feats implemented that do this sort of automatic healing after combat, and some items restore spell slots too. Feats like Die Hard would still apply on top of the short rest recoveries.
An even better, more random approach, would be to randomly check each used ability or spell slot or damage point using the Player specified percentage. So, if you chose 20% HP and 30% Spell Slots and 50% Abilities. So, you take 8 damage, 20% of it (rolling for each damage point) is not taken in the example above, and if you cast a spell, it has a 30% chance to NOT expend a spell slot, and using an ability or feat has 50% chance to NOT expend the use. This would be very exciting, a sort of Press Your Luck aspect to using spells and abilities. Shouldn't be difficult to implement either. This option alone could really let people customize difficulty and play style. Note this is not healing, but a chance of not losing the HP or spell slots when used.
Instead of charging GP to level up, allow characters to be resurrected without an item by paying GP. In this way, your economy should not be disrupted much, since in this difficult module, I would say each character is likely to die once per character level up, so you end up paying the same money, but level advancement is not blocked when short of GP. The explanation is simply that the character was not actually killed, just badly injured.
Instead of charging GP to level up, allow us to create additional characters during the game to replace dead ones. This is also where the GP that would have been spent on level up could be spent. So, it would cost GP to create new characters during the scenario, as per the level up GP costs, but don't charge for level up. I guess this would happen at certain merchants who offer mercenary services, and also available at Campfires, since reinforcements arrive, but they need to be paid!
Make it a little less damaging and more difficult to have characters "die". Allow game option to specify death HP threshold. Default is -10 as per 3.5e, but other options could be -10 per character level, or -1*max HP. It's tough enough they get knocked out easily, but even in the real world, only about 1 in 6 casualties is actually a death, and the default of -10 does not allow for that, as even at low levels, one critical hit can take you way below that. This would avoid having to reload just because a character got to -10 HP.
Another option to allow for fewer character deaths would be to give the character a Negative Level as if drained by an Undead when they would normally die, and they just fall unconscious instead of dying. Those Negative Levels could be removed in the usual ways, and as usual, if a character gets as many Negative Levels as their Character Level, they do actually die for real in the usual way, requiring Resurrection or Raise Dead. The neat thing about this approach, is you'll get those Negative Level penalties until healed, so there is a simulation of severe lasting injury in this approach.
Another possible game option to limit deaths would be to allow the character a Death Saving throw. When the character would normally die, they get a saving throw to prevent the death, and they just get KO instead.
Something I would really love is to have Is Injury Effects applied when knocked out instead of always death. I'm thinking the old AD&D injury charts by damage type, so different effects for Piercing, Fire, Bludgeoning, etc. This is a great way to slowly diminish a character's effectiveness when defeated in an encounter without needing to actually kill the character. And of course, healing spells could remove the injury effects. I think the AD&D was similar to Warhammer, where injuries had intensity 1 to 15 or something like that, and I would say an appropriately powerful healing spell would be needed to cure the injury effect. Say, something like CLW for 1 to 3, CMW for 4 to 6, CSW for 7 to 9, CCW for 10 to 12, Heal for 13 to 15. Or even better, the injury is removed by a caster level equal to or greater than the injury level. You allow spells to be cast at elevated levels already, so you could cast a CLW at level 4 to cure a level 4 Injury Effect, or a higher level cure spell would automatically cure the injury if its base CL is high enough. That also works for potions and scrolls - the CL of the potion or scroll would be used to see if it cures the Injury. I would also love to see injury effects applied both to PCs and Monsters on Critical Hits. Several possibilities here. Maybe a Fortitude save on Critical Hit (even if not confirmed) to avoid injury. Maybe a Game Option where PCs have a player specified chance of receiving an Injury Effect instead of Critical Damage on a confirmed critical. So, instead of taking double or triple damage on a confirmed critical, a PC could make a Fortitude Save (or by Game Option, always successful) get an Injury Effect instead of the extra damage. I find the critical hits are what is destroying my party the most, but they should still happen, but maybe as Injury instead of Fatal Damage.
I would LOVE to see the game keep track of how many times each character gets knocked out and killed as well, in the character statistics panel, in a list preferably including the cause of KO or death in each instance
For me, the challenge is to get through the module without a total party kill AND without reloading. So having some of these game options available, it could be possible to do that. I would love to see any and all of these options implemented