Sooo... they're just making this up as they go along, right? (spoilers for Ep. 2.11)

Not right at all. Show creators Robert and Michelle King have a proven history of creating narratives that run across entire seasons. They’ve done it with The Good Wife, with The Good Fight and now with what I think is their most ambitious project yet, Evil. I hope you enjoy this article.

https://screenrant.com/evil-season-2-sheryl-possessed-christine-lahti-response/

Of course, this kind of narrative expects the full attention of the audience, so no phones, tablets and so on.

They’re not extemporaneously “making it up.” This isn’t Lost or Super-fucking-natural.

I don't think they are making it up as they go along, I think everything you mention is meant to be interpreted by the audience but we don't have all the clues yet. In the first season, there was a lot of ambiguity about whether any of the "supernatural" events they were investigating were supernatural at all, or just the clients' excuses/explanations for bad things. I thought that was beautifully done because, yeah, even though we might think of "EVIL!" as a red devil with horns and a forked tail, actual evil is flesh-and-blood people making selfish decisions. So the first season really, I thought, played with the idea of the supernatural events being symbolic of actual evil choices that people were making.

I think in the second season, the supernatural events are seeming more as if they are meant to be literal and real: Leland's demon, for example. But there is still some ambiguity: the nun's use of cleaning fluid on Leland, Leland's exorcism (did it work? Or not?). I think there is still the idea that a person's real, evil choices, might be what produce the nightmare that is the demonic possessions. So we are still asking, are the demons real, is Eddie really alive, is the embalming fluid (or whatever it is) real, are Kristen's visions real, or are these going to turn out to be simply external symbols of the evil choices that Leland, Sheryl, and Kristen are making?

I think that all the odd "inconsistencies" that you are seeing are really part of that mystery and that the viewers are meant to be hypothesizing about the explanation for all of that.

So I think the "big picture" for the show is (a) are these supernatural events real or symbolic and (b) is each character's soul going to be saved, or will that person choose to give themselves over to evil, in the end, with Kristen's moral fate as the main question for the show.

This whole show had so much potential, but seems to be squandering it. Story after story is mostly unresolved, plot threads left hanging. I'm not saying the show needs to just wrap everything up neatly and make the show completely transparent and understandable to everyone, but we need to be given a few tidbits to keep us hanging on.