The MP3 file’s ID3v2 tag will take priority over any ID3v1 tag, as it does for most ID3 aware applications. Firefly doesn’t support multiple entries for any tag right now… but do note having multiple Genres in the tag isn’t part of the ID3v2 pseudo-standard. If I remember correctly, it’s only documented for name related values… like Artist, Composer, Conductor, etc. (“/” delimited in the tag in 2.3, or null delimited in the DOA 2.4 spec)… but virtually no applications support this functionality. (It’s better with other formats like WMA, OGG, FLAC… all handle multiple names/genres… but, as I said, Firefly doesn’t deal with it.)
The case sensitivity is a known feature. Solution? Fix your tags to be consistent. I don’t think Ron will be addressing this anytime soon, due to possible impact to low power devices like NSLU2.