I personally don't mind that the plural is foci, but it would be nice for the enchantments to be called Focus enchantments so that people realize. I mean, it's like calling it the "Gwethdesuans" enchantment anyway.
Or just make it focuses. Whatever works. But as a perfectionist and lover of good grammar, it does grate on me all the time too. :D
Crafting in general has a ton of grammatical inconsistencies. Mostly they are to do with plurals, but there's also some incorrectly conjugated verbs and dangling participles.