Op this thread really hit home for me, my very similar problems started about your daughters age, it's now a decade later and i am in a similar boat so i really feel for her, it is so difficult to accept, and isolating, and for all the rose tinted rhetoric about the NHS, for chronic and unusual illnesses it can be an absolute nightmare, and highly stressful.
Is your daughter on any medication for the pain OP, if not this would be a good place to start, she needs to tell her GP she would like a referral to your local pain management service and for physiotherapy. If she cannot advocate for herself then she can put in writing to her GP surgery that she consents for you to be allowed to discuss her medical records and that way you can speak to the GP about her directly if that is helpful.
I would also highly recommend trying talking therapy or CBT as a means to her her cope both with how she manages her pain, and how she manages her mood, in my local area you can self refer for mental health services, though in yours you may need to ask your GP to do this. It's not the answer to the problem of course, and the mental health services are very strained, but if she gets a course of CBT out of it then it could be worthwhile.
This is of course not medical advice but the biggest thing that stood out for me was your daughters symptoms regarding dizziness being upright and fainting etc. Have you considered POTS ( postural orthostatic tachycardial syndrome) or it's more common neighbour , postural hypotension - I was recently diagnosed after a decade of trying to persuade my useless health care providers that i was not making it up, after it became so bad i cannot often even sit upright for long periods. In addition my 'fibromyalgia' has recently been diagnosed as Ehlers Danlos ( a connective tissue disorder), again, after years and years of stress, pain and pushing for an answer. The average time of diagnosis for Ehlers Danlos is 8 years and i would say that was about correct in my case. Unfortunately a diagnosis has not helped in the least bit because it is ill-understood, rare, and there are no NHS specialists available other than to manage the symptoms. I am currently entirely bedbound and lacking in most of my autonomic functions at aged 30, but even with the diagnosis from clinical genetics i still visit my GP and he likes to suggest any pain i have is "probably a virus". It is maddening.
In terms of how to support your daughter i would suggest encouraging her where possible to maintain a routine, you do not want her to give up on self care. I find online forums for people with similar conditions a vital connection to the world that i lack; sometimes watching the friends i have left move on with their lives and do things i can't and am not involved with, is simply too much for me, so i find it better to be online, or finding things i can do from bed (and crucially not feeling guilty about the fact).
If she is low in vitamin D and taking a supplement, has the vitamin d level been retested? Has her thyroid been tested? I would also look to be referred to a rheumatologist to discuss her joint pain and receive a proper diagnose as to the cause, and rule out alternative conditions. I find it very helpful to provide a typed list of my symptoms and the regularity of them at these appointments or frankly, it just gets lost in listing them verbally.
I don't want to drone on -likeialreadyhave- but do feel free to PM, i empathise with this so thoroughly it has knocked me for six a bit reading it. I certainly am wishing her all the best 