I just rewatched the Lea Salonga On My Own clip, and while I don't know if Taylor could hit the notes that the song reaches when sung live on stage, the song will likely be staged/arranged slightly differently because of the differences between film and live on-stage theater, so even if it turns out that Taylor can't hit precisely the same notes one would in a live theatrical production of Les Mis, I think she'll still be able to perform the song well and without any difficulties.
I don't see how her singing the song in a movie would prevent Taylor from hitting certain notes because the song is not sung on a theater stage. In fact since movie songs are recorded in a studio, I think the vocal performance has a chance to be that much better than live theater singing.
I think the one of the things most are concerned about is the songs being rearranged to make them easier for Taylor to "handle". Those breathy barely there pop/folk vocals which have given her a shockingly great pop music career simply aren't enough for the role she has undertaken. I hope, as do others, she has something in reserve. BTW, "Safe and Sound" did nothing to give me any more confidence in Taylor's vocal ability.