If it is an official release/co-released with the original artist:
A track/recording that is a remix of another recording should typically be credited to the original recording’s artist, not to the remixer. The remixer may be credited instead if the tracklist explicitly credits the track to the remixer rather than to the original artist.
If it is an unofficial release/not sanctioned by the original artist:
A track/recording that is a remix of another recording should typically be credited to the original recording’s artist, not to the remixer. The remixer may be credited instead if the tracklist explicitly credits the track to the remixer rather than to the original artist.
Use a random number generator to invent an ISBN. (this is a joke!)
If you don’t know something, leave it empty 
MusicBrainz is interested in having artists in the artist field. So if the YouTube channel name is the same as the ‘identity’ of the remixer, then that sounds like the artist, indeed.
If it is a YouTube channel that releases lots of different remixes by different people, or uses a different name to what the remixer/artist goes by, don’t use it in the artist field (unless you really cannot find who the actual artist/s is). The channel may qualify as a label or distributor, but it is not an artist name.
Not sure… there’s quite a long thread here, I don’t think it reaches consensus but it has a bit of discussion on this topic: Release and recording name guidelance
You may also want to use @chaban’s pretty crazy list of added YouTube recordings, as a reference.