Hi,
can we say: - She'll not do it. Yes, you can.
Will there be any differences in terms of shades of meaning? It's hard to speak about such subtleties without looking at a specific context. However, very generally speaking, I would suggest that using the uncontracted form of 'not' in a place where it is unexpected serves to emphasize the word, the negative, more.
Bets wishes, Clive