>They also make heroic sacrifices to overcome their own innate weaknesses.

Clerics make heroic sacrifices? I view a Cleric building devotion as a transaction. They do some rituals, pay compliments, and in return, the Immortal lends them some power. That's hardly heroic. If anything, the bit of sacrifice may be spiritual/soul, but it always seemed akin to turning a crank to charge a battery.

>>A cleric virtually dies (enters the void) to learn the resurrection spells (altruism) which through effort (the quests) leads to MF (self-purification).

I'm not convinced any of that has to be framed as altruism. It's easy enough to picture a cleric who, desiring power over death itself, pushes their way into the void and seizes it as an entirely selfish pursuit, for example.

>Elyhaar's Feast also has the addiction component to battle, including giving up their own spirit to overcome their weaknesses in mind or body.

I take this more as a sign of how insane spiritual manipulation is. Think Bane with his Venom addiction, or Geralt quaffing potions that make him less than human. Clerics are capable of tearing part of their spirit apart to feed their own needs, and this is super dangerous. That it has an addiction component isn't a sign that it's a good thing.

>How would this differ from a paladin and their soul pool/state?

Analogies are going to get fuzzy here, so, again to be clear this is just my take on the lore (which I super acknowledge might be wrong!):

A Cleric turns a crank to power a battery.
A Paladin puts their hands in a very specific configuration to play a very specific chord on a musical instrument.

If the Cleric stops turning the crank, the battery loses charge.
If the Paladin holds their hands differently (actions?), or doesn't properly care for the instrument (soul state?), a different sound is produced, and that sound is 'Not Sanctified Soul'.

The Clerics relationship seems more like a business transaction with another being. The devotion must be true, but the Immortal doesn't care if the Cleric thinks the Immortal is a jerkbag, so long as the Cleric puts in the devotional efforts.
The Paladins relationship seems more like someone who is training to run a marathon. There's no skipping putting in the mileage, and at the end of all that training, the Paladin either put in the training, or didn't.

I'm bad at analogies.