can anyone relate to my daughter's symptoms of nausea and weight loss

Maybe do a fecal Calprotectin to see if there is any inflammation in the colon causing it?
Nausea and weightloss are often symptoms of active inflammation and IBD? Are her stools normal? Also I am assuming her thyroid levels are fine too since you have been to an endocrinologist?

Some of the symptoms you've written here reminds me of my neighbor's daughter at the same age. After nearly a year of every test possible, she was tested for celiac disease & was positive for it. I don't know if your daughter has been tested or not, perhaps something to consider.

Can I ask how much exercise / training she is doing? When you say her iron is high are you meaning serum iron, i.e. the amount of iron she has in her blood or ferritin levels, i.e. the amount of iron she has stored in her body. Both are very different.

Celiac is worth ruling out - it's a simple blood test. It's sometimes caught with an upper endoscopy, but sometimes missed on that test too if they're not really looking for it. And it causes long-term damage to the villi, something that doesn't just cause symptoms when gluten is eaten. It takes weeks or months of complete gluten free to heal. But don't go gluten free without first getting the blood test.

Has she had her gall bladder checked?

Did the bloodwork include CRP and ESR? Both normal?

For the CT Scan(s), were any with contrast (oral and or IV)?

Beave, I thought that you can see celiac during endoscopy, she had upper endoscopy and GI said all was clear with clear biopsy results. But the blood test will not hurt. CRP and ESR are normal, Ct was with the IV contrast - all normal, I guess that rules out gall bladder (also did abdominal and pelvic ultrasound - nothing). did we miss anything?
Damo123, her iron was 33 with normal range 10.0-30.0 and saturation was 56% with normal range of 15-45%, this was registered on 2 blood tests 7 months apart, endo said it's nothing, the elevation is very little, she did try low iron diet for a month with no improvement. She doesn't have joint pain but has bendy joints, lots of cracking as well.
from what she describing it sounds like motility problem: crazy air burping, food not going down and building up in the esophagus, nausea and very strong gag reflex. but she is very reluctant to do gastric emptying study - she cannot even look at eggs.
but still even with nausea she is eating around 1800 - 2000cal a day and loosing weight without any BM issues. just cannot figure it out

Delta....has she been checked for hypermobility issues.....in particular you should check if she is anywhere on the elhrs danos scale.....hypermobility can extend to blood vessels and anxiety as much as bending and cracking of joints

Damo, the endocrinologist looked into hypermobility and didn't find anything alarming. but there are still a few tests we can do which brings me hope. Thank you guys for the suggestions, I'll keep you updated if we find anything

Thanks Andreita, I am slim with fast metabolism too (55kg/165cm) but I feel that 43kg is like she is about to perish any second, I know it's my mother bear talking but still need to find out what's wrong and help her while I can