I am not aware of a single guideline for everything related to opera, but there are several separate MusicBrainzWiki pages, and a load of best practices followed by various editors.
Regarding documentation, I am aware only of one opera-specific guideline: https://wiki.musicbrainz.org/History:Style/Specific_types_of_releases/Opera. All general style rules apply as well, of course.
The rest are just best practices I usually follow, maybe you may find them useful as well.
Tracks
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Name tracks as printed on the CD cover, only correcting obvious errors. Follow the guideline referenced above.
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Assign the composer as an Artist to all tracks.
Works
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When splitting works on parts, preferrably do not rely on recordings of a particular release. Rely on catalogues or on score instead. https://imslp.org is a very good source of scores. Catalogues for many classical composers may be found here: http://www.musiqueorguequebec.ca/oeuvres2.html. Try to google for a libretto.
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If the score is not available, and the catalogue is either not available or is not detailed enough (i.e. does not show how to split an opera on parts), try to check several releases of the same opera. If all of them split on same sub-parts, apply the same splitting. But if different releases split on recordings differently, it is better to use larger parts (Acts or Scenes) and “partial recording” attribute.
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Prefer work names (including names of parts) in the original language.
Performers
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If you have time and enough information, link performers to recordings. Otherwise link them to the whole release, it is better than no links at all.
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Link recordings at least to orchestra, conductor and provide the date of recording, if known. Later this may help other editors to find more detailed meta-information, as well as identify and merge duplicates.
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For performers, enter performed roles (if known) in the “credited as” field which appears once you choose a vocal type.
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Link a performer to a recording only if you know that this performer actually performs this recording. In some cases it is printed on the cover, in other cases you may have to check the libretto. In some cases it could be useful to listen to the recording and compare it with the libretto.
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If you have a list of performers for a release, but can’t associate them with particular recordings, you may still link them to the whole release.
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When populating the “Artist” property for a recording, be careful. The script “replace recording artists” referenced in The Classical Editor Toolbox is very useful, but I believe for operas it shall be applied with care. In some operas there may be 10 or more performers on a single recording, and I feel it is not really useful to put all of them in the recording “Artist” property. I would put there only orchestra, conductor, and maybe performers of lead roles. All other performers are still credited by links, they just do not appear in the “Artist” property.
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If you edit a release entered by somebody else and do not have access to the CD cover with printed performers, you may try to find the information on discogs, allmusic, Amazon etc. Some labels, like Deutsche Grammphon, provide detailed meta-information for their releases online.
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Don’t forget to check the cover art, it may be either already uploaded to MusicBrainz, or be available somewhere (discogs etc). Sometimes it may contain additional information not provided anywhere else.
Let us take now your case as an example.
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All performers are availble on discogs release page linked to the MusicBrainz release.
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Roles for each particular recording are included in track names, so you may link particular performers to recordings.
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I can’t find the score. I have found a catalogue entry, but it is not detailed enough (it contains only the opera as a whole, but not parts). Fortunately google immediately points to a PDF with the complete Libretto where parts are indicated: https://www.telemann.org/tl_files/files/pdf/orpheus-dvd/Orpheus-DVD-Box,Booklet-Ergaenzung.pdf. Although we can’t be sure that these parts are “canonical”, that is actually indicated in score as separate sub-works, this looks good enough to me, and I would split “Orpheus” on parts according to the libretto.