Something doesn't add up regarding the connection between Amdapor and Norvrandt...

[Iscah]

It's possible that they were able to "manufacture" Sin Eaters, but that would mean many volunteers chose to give their lives to become monsters, they were converted by force, or they were using wildlife and mangling them to suit their own ends. I always got the impression that, as healers, and of the "light" they were about virtue and kindness, so the latter options two seem very unlikely.

That's the impression they give, but maybe not necessarily. The Lv60-70 SCH quests (which deal with some characters who are actually Nymian and had been in stasis since the War of the Magi) and talk about Setoto's father who had to "infiltrate" Amdapor seeking a cure for a sickness, and was caught and interrogated by the Amdapori. We don't know what happened to him after that, but Setoto believe it likely that they killed him.

No, Umbral is stasis, which is Light. Astral is creation and Darkness.

Yes, this is correct, and was confirmed in the MSQ. We are all well aware that it runs contrary to the REAL LIFE meanings of Umbral and Astral (the former meaning "shaded" which lends itself more to darkness, and the latter meaning "starlike" which lends itself better to light). In-game Light is Umbral, and Darkness is Astral.

I figure the Source's choice of "astral" and "umbral" is arbitrary - they just picked two opposing concepts to name the active and passive energy poles.

The Norvrandtian logic for naming active=Dark and passive=Light is also pretty arbitrary, except they coincidentally got it correct rather than back-to-front.

I'm pretty sure they are still 2 different poles. I'm referring to this article

It gets confusing because there are two different uses of "Darkness" and "Light" now. There are the actual forces/energies that we're dealing with at a large scale, which is what Koji is talking about in that interview (the only concept we had at that time).

Now Shadowbringers has thrown in Darkness and Light as Norvrandt's terms for the active and passive energy poles, which coincidentally line up with the aspects of actual Darkness and Light.

I remember Urianger saying something about that when he pointed at an elemental chart and asked whether we had gotten it all wrong since the people of Norvrandt saw Light as destructive and Dark as peaceful.

You're misremembering. They do question if we got it wrong, but the reasoning is different. Key quotes from his explanation:

The people of Norvrandt associate "the pole aligned with activity and growth" (Eorzean astral) to Darkness because "life's myriad colours combine to produce black".

"The pole aligned with passivity" (Eorzean umbral) is associated with Light because "peace and tranquility [are] as purest white unmarred by colour".

The conclusions are correct, but the logic seems very arbitrary.