I agree about the use of work for critique and comment being perfectly fine but the point about it being transformative when it comes to music is a weird subject, a remix would still be infringement in the case of taking samples of the original song, even if it wasn't for profit, without consent, as it is still infringing on the adaptation rights of the intellectual property owner as well as the distribution rights,if the remix was released. Although, as you've mentioned if a work is completely transformative it is definitely contestable in a court, although it's a bit of a grey area to be honest, because different people have different ideas of what's transformative.  I'm also not a lawyer but have studied music copyright law as part of my undergraduate degree in the music industry.