We know the race associated with the land of gods were the Lunarians, which were considered the gods of the red line, yet we haven't seen statues of their species on the red line, the statues we've seen were exclusively human(like), so they're probably just some members of the founder families carved out in stone to declare their new home and the reason Lily declined living up there is because the founders attempted to dispose of the other races living on the red line - successfully so.
They managed to wipe out the arguably strongest species (Lunarians), the holy knights ain't no big deal here (and whose knights are they to begin with? If they serve someone even higher up than Imu, they wouldn't have allowed Imu to literally take seat as the king of the world).
Figarland on the other hand has all the characteristics of a Celestial Dragon; having "Saint" in his name and being an arrogant human who considers those living in the lower realms as scum not worth protecting, rather you would be even worse of a scum if you protected them, also doing slave hunting and killing for sports.
I don't think your idea of Garling has a good basis here? And yeah, obviously, it wouldn't be good portrayal to the outside world if you saw the supposed "gods" the Celestial Dragons proclaim to be just being executed as trash. They have to retain their untouchable image to the outside world obviously, which doesn't tell you everything about their internal policy.
And then again, who are the holy knights answer to? Who gave them authority? And if they are separate from the WG and the Celestial Dragons, why can Garling execute Celestial Dragons? It's not like the WG ever cared to adhere to someone's rules, because again, we know they literally wiped out species.
Mary Geoise isn't Land of Gods
Land of Gods existed "Before" Mary Geoise
Meaning Mary Geoise came to Replace it, and it was Confirmed that Mary Geoise existed "Before" WG
That means it was Founders of Mary Geoise who Destroyed Land of Gods, not WG Founders.
If WG Founders Defeated Land of Gods, then They would be known as Founders of Mary Geoise, which We know They aren't
And who said that Holy Knights know about Imu?
Why is Imu spending his time alone? Just because He is Secretly Ruler of WG, doesn't mean He needs to make his entire Presence a Secret as well, after all He is a Celestial Dragon, He can simply walk around & go wherever He pleases & present himself as any other Celestial Dragon, but Secretly gives Orders to Gorosei
Yet Imu hides his entire Existence, not just his Authority, some Celestial Dragons don't even know about Void Century, how is that Possible if They are being Led by a Person from that Time Period? This means Imu hides himself even within Mary Geoise, why would that be you think?
Look, what We know for a Fact is that Garling have Higher Authority than Most CDs
I'm not saying that Holy Knights being CDs who are under Gorosei is Impossible or Unlikely but all i'm saying is that them being their own thing outside CDs who Operate similar to CP-9 in Enies Lobby isn't an Impossibility either
Being Higher Authority than Average CD doesn't necessarily mean you're Superior than "All" CDs but it's still on the Table.
And do you know what Dragon said? He said War doesn't officially Start until Holy Knights join it
And in Same Arc He said that, We are witnessing a Big Battle involving Five Elders
So this Vegapunk Conflict is bigger than the War that Dragon was teasing? It can't be
If Five Elders were bigger deal than Holy Knights then them coming to Egghead is bigger deal than Revos going at War with Holy Knights, which doesn't make sense at all from Story Perspective. It's like Kin'emon saying that War won't Officially Start until Flying Six are mobilized even when Calamities were all Mobilized, that's Contradiction
Also when Akainu was discussing what Mjosgard did, He said that Holy Knights will handle it, why Specifically them? How did He know that those Knights gonna judge Mjosgard & not Gorosei themselves do it? Same Akainu who badmouthed Gorosei for not being able to Control Doffy (Same Doffy who Blackmailed WG & Used CP-0, yet took Orders from Someone of High Authority, who till this Day wasn't Confirmed to be Gorosei or any CD)