If an artist writes “acappella” in all places where they talk about this (websites, microblogs, social media, release metadata), does that then count as artist intent? I’d personally say that it does. Maybe the artist doesn’t know any better, but nevertheless, this is how they spell this particular phrase.
MusicBrainz should (IMO) be descriptive, not prescriptive. If people are (esp. consistently) writing certain words or phrases in a certain way, MusicBrainz should mirror that.
If the artist consistently uses “a cappella” but in one instance writes “acappella”, then sure, fix it, since then it’s an obvious typo.
I don’t agree with relying on dictionaries or external sources as a rule (they can be useful to fall back on in some cases) since they, too, are descriptive, not prescriptive (at least with regards for spelling), so they change over time and never fully reflect current usage. (It takes time for a new word or a changed spelling to gain wide enough adoption for it to be notified by dictionary editors and then it needs editing and so on before it finally shows up in a dictionary.) This means that we may change something for a release tomorrow that may not be correct according to current dictionary, but may be correct according to the same dictionary’s edition a couple of years from now. Better to just have it as‐stated from the get‐go.
Yes. If it is not spoken/written/used in the context of your language, then your language is not of significant importance for how it should be used. Language is migratory and adaptive. This phrase has been picked up by the English “diaspora” and been adopted into this language. This adoption frees it from the ‘rules’ of its source/origin language. I mean, you’re using French punctuation even when writing English, so you’re already not following English grammar ‘rules’, and no one’s shouting at you for it…