Katy - it is hard, but you sound as though you are doing the right thing overall.
your dd had a hobby, which she enjyed. once it was apparent she was seriously good, you have found her (or she found - no matter) a scheme which means she can do more of it.
FWIW, if she really enjoys it as you say she does (and that is not meant ot sound doubting in any way) then don't worry she is doing too much - she will actually be relaxed by the dancing.
I was a ballet dancer. I had a scholarship to a seriously academic private school (so top marks were expected). I left school at 5pm each day (at your dd's age), went straight to my ballet school (arrived at just before 6pm - changed in the car on the way, ate tea in the car on the way etc) and danced until at least 9pm each night Mon-Fri. I needed to (not just needed to 'for my career' but actually physically needed to). All day Saturday was spent down there too.
I don't think you are wrong in stopping the choir (no one can do everything) - is there an opportunity at her school to be involved in a choir? I did choir/orchestra at school (lunchtimes mostly), as well as instrument tuition (another thing that I guess some posters would be horrified by - more pressure! but I enjoyed it, and could not give that up as much as I could not give up my dancing).
FWIW, I was not held back by not going to residential dance school young (my mum had serious reservations, especially back then (pre-national curriculum etc) at the dance/academic balance so would not let me go - I won a scholarship to white Lodge when I was 10, and having turned that down, they kept contacting me and inviting me to re-audition at just about every opportunity - at 13, 16 and 18). It can happen, and I am eternally grateful (now!) to my mum that she did not let me go so young - the things I would have missed out on, academically, woudl have been a huge sacrifice.