I would not be happy myself.
I am not a doctor but this is what I know from being in my body. First, I suggest you contact someone at the magic foundation (search for it) and get them to send you a list of pediatric neuro-endocrinologists. You don't need a regular endo, you need a pituitary endo.
1. That level of ACTH is not normal. It can indicate something else (but usually that is associated with Cushing's), however, the norm with elevated ACTH is about 300-600 with no adrenals, and over 1000 is supposed to be concerning for an ectopic source or ACTH. In my case, I have no adrenals, and I have a pituitary tumor and my ACTH is also super high - about 4000 and the docs were baffled and just suppressed it.
2. You cannot do anything about the pigmentation. As long as her ACTH is that high, it will happen. The doc can give her dex to suppress it, but they need to know the cause, if they can find it.
3. Yes IMHO. Only because typically low cortisol is a salt wasting disease and low cortisol with low sodium can cause a life threatening crisis and you don't have to tools to deal with it.
BTW I hope the CT is of the abdomen as you need a dynamic pituitary MRI to be useful... CT of head not so useful.