Guardian of Faith factors the actual damage done.
This part of your question was addressed in the Sage Advice Compendium, although in different terms:
[Q:] When a creature successfully saves against
guardian of faith and takes 10 radiant damage, how
much damage does that count against the total amount
of damage the spell can deal? Is it 20 because that’s how
much it dealt or 10 because that’s how much the target
[A:] It dealt 10 damage to the creature, so 10 is subtracted
from the total.
The question implies that damage dealt is attempted and damage taken is actual. However, the answer ignores that implied distinction, because damage taken and dealt are one and the same: both are what you are referring to as the "actual damage done"; there is no such thing as "damage attempted".
Extrapolating for there, guardian of faith accounts for the save, for multipliers (such as resistance and vulnerability), for modifiers, etc. because what matters is the actual damage done to the creature.
Guardian of faith can deal more than 60 damage.
Nowhere in it's description does guardian of faith state that is can only deal 60 damage, instead it states:
The creature takes 20 radiant damage on a failed save, or half as much damage on a successful one. The guardian vanishes when it has dealt a total of 60 damage.
For example, say the guardian had dealt a total of 50 damage so far, then it dealt another 20 damage, and now it has dealt a total of 60 damage so it vanishes. In particular, although the guardian dealt 70 damage, that also means that "it has dealt a total of 60 damage" at some point in time (but that point in time is indistinct because the 20 damage is dealt all at once) and there is no indication that you need to retroactively reduce that last instace of 20 damage.