When you are in the position where you can't turn her voice up any more.... you have but ONE direction to go.....
Turn everything else down.
Either go back to square one.....delete the envelopes and pull everything down and start over..... or .....take a short cut to get there.
When I run into this situation, I insert a new stereo bus and route "the band" to it. That allows me to keep the mix balance I have set up...envelopes, plugs, and such, and with the level control on the new bus, I can lower the instruments to let the vox shine through. Also, using compression and other such tools are fair game. I have used Cake's "Boost" on occasion to get a vox up a few more db, because what matters is hearing that voice. As long as you don't ruin the delicate voice in the process, it's all good.
Listen to my song called Give Me A Chance (
http://soundclick.com/share.cfm?id=10977017 ) I have a strong female singer followed by a 7 year old female child singing the chorus solo .... forward to approx 3 minutes into the song to hear the comparison. I used compression/boost, and production to let the child's voice stand out well in relation to the music.
That's my 2 cents and an example of what I did in this situation.