[DaveBlake]

I have tagged 95% of all my music using Musicbrainz Picard as per the various instructions in the Wiki. I'm left with a (significant) number of albums that are not in the MB database, and with a ton of albums that are now tagged incorrectly due to the inconsistencies in the MB database. For example, five albums by the same band may be tagged as Trance, Club, Dance, Electronica or Other, while in fact it all should be the same genre (trance). Picard lets me edit the genre tag (and other tags) but if I do that, will Kodi organize my MP3 files by the original tags it retrieves from MB when I add the MP3 files to the library (using the MBID tag) or will it use my edited tags?

Personally I always set the genre tag manually, other people's ideas of genre seem totally bizarre to me. Using Picard picking up genre from Musicbrainz is optional, I suggest you don't use it.

Kodi uses the tags embedded in the music files, so it will use your edited song genre values.
 

Also, if an album is not present in the MB database, can I manually edit the tags for that album (so as to be consisted with the rest) and will Kodi then use that info?

Well adding to Musicbrainz is the community spirited thing to do, but I also understand if you don't  want to spend your life doing this, maybe just do some.

Kodi will work with music files that don't have Musicbrainz id tags, mbids are not mandatory just useful - giving more accurate scraping, prevents inconsistent naming resulting in multiple entries for same artist,  and the only way to have multiple relases of the same album or different artists with the same name in the library. But say you have some music files by artist X fully tagged with Musicbrainz id, and some albums not. Kodi will match the artist by name and have only library entry for that artist with the artist mbid, but you have to be consistent with naming.

Some manual tagging tips:

• Do tag as fully as possible - ARTIST, ALBUM, TRACK, YEAR as a bare minimum
  

 

• Don't try to add some mbid tags (say for the artist) and not others (e.g. the release), there is no need and is likely to cause problems. Users always seem to get in a mess when they manually edit Musicbranz ID tags. Either let something like Picard provide them, or don't use them.
  
  
• Make use of the ARTISTS and ALBUMARTISTS (note the S) tags to help Kodi identify individual artists if the ARTIST or ALBUMARTIST tags refer to multiple artists and does not use the slash-space-slash default separator or a multi-frame tag format e.g. ID3 v2.4 or Vorbis.
  For example in ID3 v2.3 tags format
  ARTIST = "Craig Chaquico (with Russ Freeman)"
  ARTISTS = Craig Chaquico / Russ Freeman
  
  
• Be consistent with artist names - "Peter Tchaikosky", "Tchiakovsy" and "Pytor Iilich Tchaikovsky" will be taken by Kodi as 3 diffferent artists unless there are Musicbrainz tags showing they are the same. In that case Kodi takes the first artist name it scans as the definaitve one for the library entry.
  

Since you have a "humongous collection of music" can I also suggest that you try splitting it into multiple music sources when adding it to the music library. It will not only mean that you can try out how the library works on a smaller subset, but also give you another way to browse your music in more accessible chunks. A feature in v18 onwards.