Nah I'd say ENFP... think of some of the concepts in her lyrics on the earlier Soviet Kitsch. Pavlov's daughter is riddled with complex ideas. Newer stuff is way more cutesy and mainstream... but that sells right
I have a hunch that Regina Spektor is an awkward eSFP. She seems to have this quiet sort of bubbliness in persona, alongside this sort of softness to herself. She appears to focus more on her physical surroundings than what is metaphoric and is inspired by what is concrete, also seeming to project that xSFPish "make-it-up-as-you-go-along" sort of attitude. Although I could see her as an INFP to an extent, I still don't see as much Ne in her as I do Se, as while Ne tries to interpret hidden meanings and focuses on what could be, Se is constantly playing with reality and experiencing the present moment. To me, Spektor seems more inspired by what she sees in physical reality that trying to create abstract meaning to her music, as Ne user likely would. She herself described her music as non-sensical/ having no meaning (jokingly) in an interview, and also seems to just want to express what she feels physically, no matter how simplistic or complex her thoughts may be.
I would agree with her being an ISFP as well, but I just sense more dominant Se from her than Fi, and the fact that she certainly isn't very outgoing/ assertive wouldn't necessarily make her an introvert. Somehow, she even reminds me more of a young child just playing with what they see and showing curiosity in what they hear. I agree, as well, that her intuition is introverted, tertiary or inferior.