I've been reflecting on your thoughts here, and this is what I've come up with fwiw.
You are right that the iNtuition she uses is Ni. It is also interesting to see that her use of reconciling dichotomies has become more effective as she has matured. Her early attempts are somewhat arbitrary and clique. This suggests to me that it is not her primary function, but is in there some place. I guess what causes me to doubt the INFJ profile for her is that her Sensing seems much stronger than her Thinking. Her Thinking function is nearly absent in her work. I also find her Feeling to be more introverted as well. It is about her deep, intense inner world. The following two types seem more plausible to me. I lean towards thinking that she does in fact use Ni, but it is subjected to her Sensing, which is most always a dominant theme in her lyrics. She is bright and likely developed her Fi and Se early on, so you do see the tertiary function present as well. What do you think?
ISFP - Fi, Se, Ni, Te (most likely)
ENFJ - Fe, Ni, Se, Ti (less likely)
IxFx types can be especially difficult to distinguish between when three or four of their functions are highly developed.