There is a different reference range, because adults produce more IGA than young children.
If her total IGA is below the range, then yes, she is IGA deficient.
My 4 year old has a total IGA of 21, but her age-adjusted range starts at 27. Her pediatric GI considers her to be IGA deficient, and therefore the celiac panel is not a valid test for her. It's true that they can run some IGG tests, but it is not that accurate for celiac.
Honestly, I think it's time to take her to a GI (preferably one who specializes in children). Failure to Thrive is a good enough reason on its own, but especially with the constipation. My experience is that too many pediatricians try to handle big problems on their own, without referring to specialists, but many issues are really outside of the scope of their expertise.
I would not try a gluten-free diet without first seeing a GI, and seeing if they want to scope her.
Good for you! I totally gave up on my ped. I hope you get an appt soon. Make sure you tell the receptionist he is having black stools and there has been visible blood in his stool. If there have been any weight gain issues, mention that too.
And if it's still a while before your appt, ask if they have a last minute cancellation waitlist, or if you can periodically call to see if anyone has cancelled.