"I wonder if Christina Bianco got some serious consideration for the role."
Doubtful. As talented as Bianco is, she's almost completely unknown outside of the theatre world.
Feldstein has a film career, in addition to earning good reviews for her featured turn in a huge Broadway revival not that long ago.
Plus, she's the best friend of current It Boy, Ben Platt, and their close relationship will magnify her to his considerable fan base.
However, it would not surprise me a bit if Bianco was hired to stand by for Feldstein. She'd be exactly what producers would want to stand by, a huge talent who's played the role already and could step in at any moment and offer a Broadway audience a terrific performance. And, since FUNNY GIRL has pretty much fallen out of the repertory, there probably aren't a lot of actresses of that caliber with Fanny already on their resume who could walk in off the street with little or no rehearsal and give a performance.