first off i must say i aboslutely adore Jesse, and feel out of all the girls left she is the ideal choice for nancy, by far one of the best singers left in[if not the best], her acting ability, when seen, has been good [the accent can be worked on]...but...
the key to whether she gets the part will come down to her dancing and movement, ultimatly her tomboyish nature would genrally work well in the roll apart from a few scenes, remember she has been working the streets for years so may require a more ladish nature to her character.
but having said that they will be times where she need to appear more feminine and gracefull [tho not a princess; think 'working girl rather than lady of the manor] but obviouslly at the mo Jessie is too much of tomboy one there are several ways to deal this is firstly someone should stop putting her stelleto's, she obviously can't walk properly in them YET!!, start her off in small heels as this would not make her awkward tomboyishness so pronounced and she would then gain conferenced to move more freelly.