It's fine, but 5e class names all follow the general "rule" that they're just one word. It's "Paladin" and "Wizard", not "Holy Knight" and "Magic-User". That's the main reason why I personally prefer "Arcknight" over "Arcane Knight". "Arcane Knight" just feels clunkier than the other class names than "Arcknight", IMO.
Yeeeep. Stylistic coherence is part of what makes a game…work. It’s why spin-off games like Adventures In Middle Earth or Heroes of Eiru and such have a distinctive style but keep the basics in a style that doesn’t
read differently from 5e.
So Arknight for a class, Eldritch Knight for a subclass. Why? Because it’s part of the D&D style guide, and has been since what, 1e ADnD? Earlier?
That doesn't seem to be any worse than openly stealing the tools of currently-living religions for use as weapons of war (when, as I am given to understand it, most Wiccan traditions would openly reject such violence).
While I have no problem with Janisary as a subclass name, I don’t think that using a variation of atheme, which came into use in the works of Alastair Crowley, IIRC, and is used by many traditions, from wiccans to satanists, is a problem.
Edit: just some notes on the atheme. It’s a term that precedes Wicca, and even Alistair Crowley and his cult, or the Hermetic Order of The Golden Dawn, where he developed his earliest practices. He took it from dusty tomes, which had recorded it as one of several older ritual tools found in the Lesser Key of Solomon, and Wicca appropriated it, along with many of the ritual technology used in modern witchcraft.
The names I listed along side it in the OP, like artavus, come from the same root as atheme and are also terms for a ritual blade, or at times a blade used for craftwork.
As for Wiccan pacifism, it isn’t a universal truth of wiccans, and especially not of everyone whose ritual practices involves the tool as such, ie by that name. Not only do some witches use a
sword, rather than a knife, many who use the atheme practice fighting sports (often reenactment based, like SCA and some HEMA groups and offshoots), serve in the military of their country, and
don’t believe that justified violence, such as self defense and punching Nazis, is a problem.
Further, it’s original usage as a ritual tool was recorded in books in which it was used in ritual bloodletting and sacrifice.
So no, Wicca does not own the atheme, nor any other basic ritual tool.