I think she'll do pretty well in the role. We know she can act the hell out of it, so at least we'll get half of a good performance.
Plus, is Mrs. Lovett really that dificult to sing? I mean, for this movie, where there are bound to be cuts (I would think WAIT is gone for sure) and the fact she only has to sing it once in a studio (where computers can help her a bit), I'm not too worried about her singning.
I am a firm believer in serendipity- all the random pieces coming together in one wonderful moment, when suddenly you see what their purpose was all along.