I have never seen video footage of Andrea as Fantine but awhile back there was a video on youtube of an "Entertainment Tonight" segment focusing on her joining the cast and it had some small clips of her singing during rehearsals.
As for Alexis as Young Cosette, there was a teeny tiny snippet at the end of the original "Turning Point" if anybody has that and can upload it. She sang "Castle of a Cloud" on Andrea's PBS special but not in costume or anything.
Andrea is playing Fantine and Alexis is playing Eponine this fall at Ogunquit Playhouse so maybe there will be some bootleg footage of that.
Lastly, that 4NY commercial with Alexis on it plays ALL THE TIME here. The TV spot is actually an edited version that doesn't have her face on it but I watch "Today" every morning when I am getting ready to leave and I always hear the "There's magic insiiiiiide" part. That's Edd's theme song.