It sounds as if you're describing a party above who has concluded that Gygaxian labels of good and evil are incorrect
It goes deeper than that.
Those definitions, and some of their later-edition counterparts, use concepts like "dignity", "rights", "innocent", "truth", "beauty" etc.
So what does a Detect Good spell latch onto? Does it latch onto a character's attitude towards, and treatment of, truth, beauty and human dignity? But in that case, who gets to determine that the Seven Heavens rather than the succubus has really got the hang of beauty and dignity? In a broad-brush or four-colour game we can take it for granted that the Heavens are right and the succubus wrong, but that won't do for a more subtle game.
In practice, at least as I've experienced it, these spells latch onto the GM's two-word alignment descriptor for the character in question - but that's hardly making the tension go away, is it?
Part of the issue is that "Good" in D&D isn't meaningful beyond a few basic facts. You can say that someone is "Good" becaue you magic it and you can say that Good people tend to act like X, Y, and Z
<snip>
If someone were to go to Celestia and Detect Good and fail to get a ping, it would mean that the area around them doesn't have traits X, Y, and Z that are defined as Good by the mutliverse (ie, the rulebooks/gygax/crawford, or in PS specifically, a sort of planar consensus).
It would probably mean that - for whatever reason - the beings who live there and who make up the plane's matter are no longer doing the right thing by society, helping others, or acting according to conscience.
This doesn't really address what, in post , I called the first issue with Detect Copt.
You seem to assume that whether or not someone is beahving in ways X, Y and Z is epistemically accessible in just the same way whether "X", "Y" or "Z" is
stopping at traffic lights and
shaking hands as a greeting and
not leaving the house naked, or
doing the right thing by society and
helping others and
fostering and admiring beauty. But as soon as you move out of broad-brush or four-colour storytelling this isn't the case. There is no sort of category error or basic failure of comprehension, for instance, in a succubus arguing that Sadeian libertarianism is true, and hence that by breaking down received mores she is in fact doing the right thing by society. (In fact, when you compare contemporary sexual ethics across huge parts of the world, compared to sexual ethics when the succubus was first conceived of as a threat to human wellbeing, you can see that the Sadeian argument has already made great progress!)
But if she is right, then Detect Good should ping on her, and not the Seven Heavens,
even though the basic facts of their behaviour are no different from those set out in the relevant rulebooks. Which, as I said, would be verging on the incoherent.
Two solutions suggest themselves. First, stick to broad-brush or four-colour rather than subtle explorations of value. This is the norm for D&D, I think, but by no means exhausts the possibilities inherent in the system. Second, drop alignment as an ingame phenomenon directly accessible via informational/cosmological magic.
I'm arguing that alignment is am objective phenomena, which like all things objective, is subjectively interpreted according to the value system of the observer.
This seems to assume a contrast between
objective things and
value systems. But alignment is about
values. Even if you treat "good" and "evil", as they are used in the alignment system, as shorthand labels rather than having their ordinary English evaluative meanings, you don't avoid the problem that the
X,
Y[ and
Z for which they are shorthands are, as I've just replied to KM, themselves open to argumentation and contestation.