Oh gosh Rosa, you are a bugger springing that on us like this. I was quite involved with Basil there.
I think you are quite right,though - but not sure it's to do with JA's youth. Well it is, but more perhaps to do with the fact that the form of the romantic novel utterly fails her youthful anger.
S and S's end is ambivalent, to say the least, I think. Brandon, at best, is an unconvincing sub-hero - there will always be question marks over his (inappropriate?) relationship with Marianne, and the extent to which she has made the best of a bad job, what with being damaged goods and all.
Equally, Edmund is TOTALLY compromised as a hero. With him, JA is telling us very clearly that Romance - and perhaps the novel - will never fulfill our expectations. We can never be sure, I think, is the lesson.
All these issues have been suppressed by the time she gets to P and P, I think? though they still peep through.