A few scattered responses.

RE: Erratic Errata's native tongue: It seems to be non-English, and the general consensus seems to be that it's French, but I don't think they've confirmed or denied it.

Re: Referring to Black by appearance: He is, as far as I know, the only one consistently referred to like this. This is in part because his appearance IS unique. The green eyes in particularly are seen as incredibly creepy and unnatural by Praesi. It might be easy to forget, but the Praesi are black, and tend toward African features, skin, eye, and hair colors. Amadeus has a more European appearance due to his ancestry, so him having pale skin, green eyes, and black, straight hair is unsettling to Praesi. He is almost always referred to or thought of as the "green eyed man" by people scared absolutely ****less of the way his eyes seem to glow in the dark and pierce right through them. There's also probably some symbology there involving the usual connotations of "green eyed" meaning jealous or grasping; Black is intensely jealous of how "easy" the side of Good has it and has made it his life's purpose to spitefully throw the game board to the ground because of it.

Re: Good vs Evil: It gets more muddled as the books go on. The tail end of book 3 and the start of 4 in particular start to show that William "I'm going to brainwash a bunch of innocent people into my unflinching army against evil" Lone Swordsman (whatever his actual last name is) is not an outlier when it comes to Well Intentioned Extremism among the the Good side.

Re: Nations that don't follow the Gods: Keep in mind that Praes DOES follow the gods; the Gods Below. Look to Callow for a Good aligned nation that just kind of pays lip service to the Gods, or to a lesser extent Procer.