It seems the question is more about how omniscient the entity/philosophy empowering the Paladin is, and how much or little action they take.
The Paladin has been deceived and is about to strike down an innocent person. Would it not be more in keeping with the tenets of LG for the Power in question to intercede to prevent that innocent's death than to punish the Paladin immediately afterwards?
This is a really good question IMO.
In brief, my answer would be, "Whether or not it fits with your conception, it doesn't seem to fit with how narratives about transgressions usually work."
In narratives about transgressions, the 'power' almost never acts to stop someone from committing the transgression. There could be a variaty of explanations for this:
1) If you intervene to prevent the transgression, you are basically violating the free will of the person's involved. It could be argued that this would be a worse crime for a diety to commit than transgression being commited.
2) It could be that the LG diety sees the chastisement, atonement, and redemption process as being the lesser of several evils it is forced to choose between when his servants are misbehaving. An example would be Arthur's mortal combat with Lancelot in the movie Excalibur, where Arthur's transgression is ultimately made to serve good - humbling both Arthur and Lancelot and bringing them into friendship.
3) In the case of dieties which are explicitly not omniscient (like most D&D dieties) it could very well be that the diety easily misses an act of impending evil amongst all it must keep track of, but quickly becomes aware of actually committed evil - the blood of an innocent crying out from the ground, for example.
From a practical standpoint, direct divine intervention used all the time is going to be unbalancing and railroading in a game.
That isn't to say it shouldn't occasionally happen. As a DM I'd be evaluating which made for the more interesting story, and which course of action was in keeping with the deities personality. I'd count it against the player's divine interventions though (I have formalized rules), meaning additional aid might not be as forthcoming for some time.
The difficult question, to me, is whether the Paladin's powers can be used as a deception detector "Whoa, guys, something is wrong - my Lay on Hands isn't working! Something we are doing must not be as righteous and benevolent as we have been lead to believe!" As I don't want to eliminate the potential that a Paladin could be deceived, I think that forces acceptance that he does not lose his abilities if he commits an evil act accidentally or unknowingly, outside of gross negligence or wilful blindness.
I generally agree, but only because you added the 'gross conduct' clause. When my players running a Champion (homebrew 'Paladin' class) commit minor transgressions knowingly or especially unknowingly, I typically don't deal with this by way of loss of status immediately. Rather, the Champion recieves warnings via dreams or other omens that something is wrong with his conduct. These become progressively worse if he doesn't correct the situation, and might ultimately lead to loss of status if ignored.
But gross transgresssion of the code, so that the player IMO ought to have thought twice about doing something so rash, would draw loss of status - albiet with a chance of 'simple' atonement. If for example my player's current Champion of Aravar (the God who protects traveller's on their way the after life), was tricked into doing something rash like waylaying and killing an innocent traveller or participating in an act of piracy on an innocent merchant or defiling the bodies of the dead, that would draw immediate censure even if the character thought he was doing the right thing or didn't realize what he was doing. In fact, it would draw immediate censure even if he'd been dominated or charmed into doing those things.
In situations of doubt, I'm inclined to rule he keeps his powers, but that doesn't mean his superiors in his order, his religious leaders or the general populace has to extend the same benefit of the doubt. Maybe a month of scrubbing outhouses, or hearing the village priest admonish his parish to "Heed the example of CharacterName the Foolish - do not walk in his footsteps" might cause a bit more attention to be paid in the future.
Heh. Not all atonement need be some vain glorious deed like killing a dragon.