An ISFP's outlook on events could be described as deepening and internal. An ENTP's could be described as broadening and external.
ENTPs are known for their living life in the future. They always have a vision for tomorrow. They are thrilled by ideas and possibilities. Did Juno seem to have this sort of outlook on life? Not from what I could see. Juno seemed to take her external situation for granted and concern herself with her inner feelings about her situation. A movie about a pregnant ENTP would have been sooooo different. It would have been more external, the tone would have been completely different (much more jovial), and we would have seen more people and places. What we did see was a deepening internal setting. We saw introspection. We saw characters coming to terms with their feelings. We heard pensive music. How, in the very least, could this not be introverted? Secondly, how is this introspective, reflective.
Neither Juno nor the movie was very philosophical either, and that's a big N giveaway. It was about a common struggle, teenage pregnancy, and one quirky girl's emotional battle with it. Juno was anything but idealistic, as Ns, and especially ENs, tend to be. She didn't try to make any grand statements about life or her pregnancy. She took part in the struggle, taking the situation itself for granted.
ISFP leads with Fi, ENTP leads with Ne. There is a distinct lack of feelings from Juno's side (an almost flippant attitude like, 'well, shyte, this sucker is real....well, now what?'). An ISFP would be Fi-driven in their reaction, which Juno was not.
Even in the ultrasound scene, Juno comments, something along the lines of why she can't understand why people get all teary-eyed and cry when seeing ultrasound because it just looks like an alien, or something.
Your bolded is pivotal part of the story, a character that is so anti-thesis of emotions...finally coming to terms with it. Not a Fi primary likely.
That was a huge story-arch, character development. Juno, who is so laisser-faire with feelings, culmunates in the final scene, after the birth, as Bleeker lies with her on the hospital bed....and she finally, cries.
She rarely looks "in"....not with her feelings with Bleeker (she told him that they had sex out of boredom), not with the overstepping of bounds with a married man, not with what to do once she found out she was pregnant.
The most rational choice to her was - I'm a kid, I can't be a mom. So get rid of it.
And, when she goes to the abortion clinic and meets her picketing anti-abortion friend outside, it doesn't phase Juno when the girl tells her that the baby has a hearbeat. But, a random comment, 'it has fingernails', makes her pause.
Which...becomes the symbolic reason she opts out of the abortion. She sees the 'reality' of fingernails everywhere - from another woman at the clinic clicking her fingernails, to another who's playing with her fingernails, another who's putting on nailpolish (and for us audience, as an ironic humour - her step-mother is a nail technician). THIS "random" (Ne-connected + Fe) reason makes her opt out of the abortion, when we the audience, know it's just a symbolism for her anxiety...which she herself can't even place or identify (shadow Fi).