My amnio showed trisomy 21. I don't want to abort but I don't want a csection if needed. Can I say that?

Delivery up to you. You should talk now with your OB about the route of delivery. Stay open to c-section if your life is in danger.
No. If you did you could be charged with child endangerment and go to prison. Either be all in or all out for a special needs child. If you try to make a self serving decision like withholding treatment that you would not for a normal infant you would be charged in my state.
You need to. Discuss with your ob. Failure to normally progress in delivery process may cause not only the foetus/infan trouble, but it may not be extractable from your womb through your birth canal. C-section is less pain and truama, so please air your worries with the obstetrican and be sure of consequences.