It's true that Matey is slightly more developed than Frau Mieders or Mlle de Lachenais, but a lot of this is IMO offset by the fact that she is sometimes Matron Gould and NOT Matron Lloyd, sometimes the other way round, very often only one or the other seems to exist (she kind of seems to expand and contract in line with school/plot requirements, a bit like Freudesheim). The gesture about calling her Gwynneth for a bit after her sister dies ought to be touching but it grates on me cos it's Joey because actually some people do already call her Gwynneth before this! In fact Nell calls Gertrude Rider 'Gwynneth' at one point in Oberland, which I don't think is entirely unconnected to the magical Karen-is-always-there assumption - not awful, but somewhat revealing!
On the "two little n- boys" incident: the joke, in Joey recounting it, is that they accidentally look like black boys, isn't it? In a way that wouldn't be as funny if they were green, because nobody is green. I'd say a comparable version would be for example if they had covered themselves in something yellow (Joyce's sulphur cakes?) and Joey had described them as "two little Chinese boys!"
I do get that it's of its time, but I also think if I were a black child reading that I would probably feel gratuitously hurt. So if I were editing them for the modern child, I would probably think very seriously about taking it out, and I'm comparing that with Armada who took all kinds of bits out seemingly somewhat at random. If I were GGB, I don't know - harder to say. I do wish GGB would modify "working like a n-" with an explanation in their notes about it, and tbf although they otherwise 'don't modify' the text they do in fact correct "obvious errors" and standardise hyphenations etc and explain that in the notes.
Mostly though if I were editing I just wouldn't bother reprinting Joey Goes because it's crap. 
The bit where Joyce and co dress up as savages is different, IMO. I can't really justify why! I suppose partly because it's so critical to a major plot. I get that all of these issues are on a continuum and I wouldn't want to fully sanitise the books - it wouldn't work, anyway - but I suppose the n word is just so obvious that I'd personally chop that out. That said, I'd still hate my child to read it and think 'savages' was an alright word to use, or that the concept used here is acceptable... I don't know.
Who the bloody hell is Evan Evans if he's not Griffith Griffiths, look you? I'm off to look him up in the encyclopaedia now. Coming, Nell?