According to the skillweb diagram, Lowmagic is needed for totems and Highmagic for rituals, yet this isn't actually the way it seems in the game? Is this a bug, or a mistake in the help files? :/

Has to be a helpfile mistake, since the "Ur'guard can take herbs" etc argument has been well established and is clearly part of the economic balance. Since we're required to take rituals, requiring us to also take lowmagic would disrupt this balance.

Actually I believe you are missreading the skillweb tree. The Webtree explains the origins of skills and thier lesson types, not what skill is depandant on another.

The skill web is sort of unintuitive. What that means is that High Magic is the parent skillset of Cosmic. Increasing your High Magic abilities will also give some improvement to your Cosmic abilities. "Parent" should not be confused with "specialization". Aquamancy *is* Elementalism; it is a special form of Elementalism. But to say that Aquamancy is distinct from Elementalism is not entirely correct. Aquamancy and Geomancy are simply two variations of the same skillset: Elementalism. On the other hand, High Magic and Cosmic are two totally different skillsets other than the one flows from the other, and improving the parent will improve the child. How does it imprpove things? It varies. Maybe you do more damage, a spell lasts longer, or maybe it gives you better odds in an ability with a randomness factor. Improving the parent of forging will make your items have longer decay times.

Really? So if I learn planar, my demesne or staff or whatever will be better?

I knew there was some other reason I was transing lowmagic besides Serpent.

marginally, yes

I think the demesne goes faster off it, yes.

No, they said they changed that aspect, at least.

Now that I think about it, didn't Estarra or Roark or someone say at one point that they were going to do that thing orignally with the parent common improving the child skill, but scrapped it?

No.

So since Combat is the parent of Knighthood which is the parent of Forging, someone who's transed all three will have longer decay rates than one who's Inept Combat but transed the other two?

A parent skillset only modifies its direct children. It will not modify its "grandchildren".