OP i haven't read the full thing but I just wanted to hopefully give you some pointers.
The symptoms your daughter has are scarily like me. I was poorly, always sick and constipated and had diarrhoea from ages 3+. Had problems all the way until teenhood. Had X-rays, enemas, laxatives, and would just poop water but it would be the overflow of what was impacted. I've had several manual impacts on removals at hospital I couldn't even tell you.
I had a CT scan at 23 years old over something completely different. They found by total mistake that I had a maltorated bowel and intestine. My organs are all the wrong way round and I was born with it. But for some bloody reason it was never picked up on X-ray.
I had a prosecute called the LADDS procedure at 23 (was having cancer surgery and treatment at the same time, not on my bowel, but they combined the two issues and did it after the surgery)
They moved my organs to the 'normal' place where they should be. Trouble is, they lost ALL muscle memory and literally shut down. Then I started having all the problems mimicking my childhood.
I'm not saying this is what your DD has, but I completely understand your frustration. You know that feeling when someone says to you 'have a ginger biscuit' when you've got morning sickness? I got that when people would say 'try a probiotic, try prunes'
These things will not help a chronic digestive problem.
Things that have helped me long the way:
Phosphate enemas. Can get them on prescription or OTC. They're not the nicest but the relief after is so worth it.
Have a small footstool by the toilet. Your natural position to birth and poo is squatting. Having a stool to help you in that position on the loo works WONDERS and it's so simple.
If things get really bad, I take picolax. It's the strongest laxative that's available and normally used to do a total clear out, quickly, for surgery or something like that. Picolax will work. But it's aggressive and dehydrates you. But it will move the inpaction.
I don't have any other real pointers and I don't know if that can relate to your child but I totally understand your frustration.
I'm now 28, my digestive system has sort of moved back to the wrong place and is working okay.. not perfect but okay.
Get a stool for the bathroom and some phosphate enemas.
*one thing to point out with enemas, suppositories, the likes, these will ONLY work if the poo is low enough in the bottom of the bowel and colon. If the blockage is higher up and in the intestine or top of the bowel, they won't help. That's when you will need picolax / movicol (movicol is much less aggressive) to hit it from both ends. But be careful if you do this and make sure she drinks a shit tonne of water as it can dehydrate her easily.
Hope your daughter feels better and keep pressing with the doctor. Get a second opinion and try and get them to CT scan her. They can see a lot more on a CT.