1. Confusing composer with artist. If the song is a remix, the artist is the remixer. The original artist is the composer.
No, the original artist is the original artist (ID3v2.2 tag TOA, ID3v2.3 tag TOPE). They may also be the composer, but that is by no means certain.
2. Genres are fuzzy. Lots of songs fit into many genres. Picking a single genre is inaccurate at best. Sadly the id3 spec only lets us pick one, so I comma separate them out of protest.
No, the ID3v2.2 and ID3v2.3 specs let you select multiple genres, or even mix multiple pre-defined ones with a custom one. One thing they do fall down on is defining how to tag a song with multiple custom genres.
3. The infamous "Various Artists" artist. Likewise with genres, I comma separate artists because the id3 spec doesn't let us add multiple artists.
No, ID3v2.2 and ID3v2.3 specify that multiple artists should be separated with a / character.
It may be that the vendors don't follow the spec, but you're being very unjust in blaming the authors of a spec with you obviously haven't read.