Exactly.I think you're generalizing the characterization of what 'T' constitutes. Logical rationale happens irrespective of dominant trait expression -- emotion is a logical pattern of instinctive event spurred by a mechanical connection to environment.
IIRC, Juno's rationale for not getting the abortion was likely an Fi event -- she was exceedingly uncomfortable in the clinic, and didn't want to go through with the procedure on this basis.
While I agree that Fi was likely in charge of her reasoning here, I don't think it necessarily permeates her cognitive being, as one would otherwise expect of an ISFP. We don't see Juno displaying 'stereotypical' woe-is-me behavior, which could be argued as the sort of resilience one would normally find in an ExTP when dealing with an event as profound as teenage pregnancy. Her flippant treatment of Bleeker is also an ExTP trope, as is her intellectual infatuation with the hobbies of Mark.
Admittedly, we're in kind of a weird, nebulous space, as we're dissecting cinematic characters that might not conform to the sort of behavior otherwise anticipated against type. At best, I'll concede that Juno could be an ExTP, versus ENTP...
While I see what you're saying here (ISFP = interest in art), I think you do your reasoning a disservice here. I daresay we could argue just as easily that the 'Hamburger' phone is an example of Ne connection. Or, that her obsession with fake tobacco pipes / weird oversized sunglasses isn't also an example of Ne in motion.
Not much can be concluded from her interests in art. Art is generally a conceptual entity culled from a realistic baseline. (Munch's 'The Scream' as an expression of hopelessness and depression he felt from personal alienation/his array of mental illnesses).
As art flows 'twixt N and S realms, it's a tough nut to sell as evidence.
I don't think offering personal behavior as evidence (to your bolded point) makes much sense. As a counterpoint, I could simply retort that I see much of myself in Juno, and she is therefore ENTP as a result...
Edit: And yes, improvisational is a word (a pretty common one, actually).