Sorry you are having to deal with your daughters health issues with her heart. I have no real suggestions on her heart problems but would recommend that you seek a second opinion at a large pediatric center if you feel it would help you understand her condition and get more information.
I do however have a suggestion with her sleep apnea, if you have not already seen an ENT then that would be a good step to take, they can evaluate if something simple like enlarged or generous tonsils and therefore adenoid tissue may be contributing. A sleep study would also be helpful, in order to monitor how much sleep apnea she is having and understanding it a bit more. I presume you are probably using a sleep monitor too? I imagine that a downs syndrome support group may also be a good resource for you, also her therapists will probably have some good information too.
Best wishes and I hope you get some further direction and progress soon.
Fiona
I am not a pediatric cardiologist. The only people I see with congenital heart disease are adults, most of whom have been corrected. Sorry I can't be of more help.