Hi Lesly!  I think you need to do whatever it takes to get the injection for your child.  I have 2 daughters, 7 & 5 years, with SSW CAH.  When my first daughter was born her endo thought we did not need the injection because we could get to the hospital in enough time.  I did some research, like you, and decided she DID need us to have it.  He still did not feel "comfortable" about it, so I decided one day, we would go to the endo to get the shot and learn how to use it.  I called ahead to let them know we were picking it up they gave me their opinion once again, I told them I disagreed and would be there in 20 minutes to get it.  I showed up with our daughter.  Every 5 minutes I checked with the receptionist to see if the script and our "our shot lesson" were ready.  After an hour or so they realized I was not giving up!  We left there that day with the injection, the tools to give and the knowledge of how to do it.  Sometimes you just have to be like that.  I ended up firing him and going to someone else in his practice but word got out- Abby’s mom is no one to mess with.  She will protect her no matter what.  You need to be the same.  Severe salt wasting or not- crisis and dehydration CAN happen.  you need to prepare for the worst and expect the best! 
We recently had a similar situation where the endo didn’t think it was necessary for our daughters to have solu-cortef at school.  I disagreed, he disagreed, my point was stronger- I am the mother and protector of these girls!  They cannot yet do this for themselves and I am in the "better safe than sorry camp".  I hope we will never need to use it at school but I feel much better knowing it is there. 
I often wonder why endos have so much difficulty letting the moms and dads participate in the care of their patients.  It’s as if "turning over control" is too much for them.  Although in reality, we generally have much more hands on experience with CAH than even the best endos! 
Please do whatever it takes to get the injection and learn how to use it for your child!  It is so much better to live without regrets!  Best of luck!
Josie