[RPG] How do the Warding Bond and Sanctuary spells interact

As of the 2018 errata, if an enemy takes damage from your spirit guardians spell, your sanctuary spell ends

The description of the sanctuary spell (PHB, p. 272) originally said:

If the warded creature makes an attack or casts a spell that affects an enemy creature, this spell ends.

However, the 2018 PHB errata added "deals damage to another creature" to the list of conditions that end the sanctuary spell. The description of the sanctuary spell now reads (emphasis mine):

If the warded creature makes an attack, casts a spell that affects an enemy, or deals damage to another creature, this spell ends.

Even though the spirit guardians spell doesn't directly affect an enemy creature when cast, it does deal damage to them:

An affected creature's speed is halved in the area, and when the creature enters the area for the first time on a turn or starts its turn there, it must make a Wisdom saving throw. On a failed save, the creature takes 3d8 radiant damage (if you are good or neutral) or 3d8 necrotic damage (if you are evil). On a successful save, the creature takes half as much damage.

As a result, the sanctuary spell ends as soon as an enemy takes damage from a spirit guardians spell cast by the warded creature.

Even before the 2018 errata, the intent was that damaging an enemy would end the spell (though this was not reflected in the description of the spell). In an unofficial tweet in July 2016, rules designer Jeremy Crawford stated:

You first cast spirit guardians and then sanctuary. Does sanctuary end when spirit guardians deals damage to enemies?

RAW: Sanctuary isn't stopped if an already-cast spell deals damage. RAI: A damaging activity ends the spell.

RAI means errata or a future edition would possibly change RAW, right? Thanks for the answer

That's correct

Clearly, the 2018 PHB errata changed the spell so that it now works as intended.

The short answer is that your Order of Operations is correct for both the attack and the Fireball. The rules use the phrasing "takes damage" consistently throughout.

The example given in the Damage Resistance and Vulnerability section (page 197 of the PHB), answers a lot of your questions.

For example, a creature has resistance to bludgeoning damage and is hit by an attack that deals 25 bludgeoning damage. The creature is also within a magical aura that reduces all damage by 5. The 25 damage is first reduced by 5 and then halved, so the creature takes 10 damage.

So, the raw damage is phrased as "an attack that deals 25 bludgeoning damage". Then all the calculations are done, then the final result is phrased "the creature takes 10 damage". Warding Bond says that

each time it takes damage, you take the same amount of damage.

Therefore, the Cleric will take the actual amount of damage that was applied to the Paladin's hit points after all the resistances and saves were calculated.

Fireball won't change the order of operations, since the Dexterity save determines how much damage the Paladin takes. For proof, we can go to Fireball itself:

A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.

Or, to put it differently, a target takes half as much damage on a successful save.