So she's got post viral fatigue, which has led to ME. This is very early days and I'm surprised she's been diagnosed so quickly and is very different from my experience. As I said, age is on her side. And everyone responds differently to different treatments. I'm not trying to give you hope. I'm relaying information I've received from the various people I see.
Firstly, is your mother prepared to pay for all the osteopath treatments? If she is, I'd have a think about the osteopath. Is this a osteopath, who does cranial osteopathy? I have Bowen therapy, which is similar, that is to say, it has a similar effect on the body. It helps my body to relax and also helps with my pain as I also have chronic pain. My body struggles to relax and sleep well and deeply, Bowen therapy helps with this. If the osteopath is offering non invasive physical treatment, I would take your mother up on that offer.
ME/CFS people often can't metabolise the vitamins we can buy from the shops. They use less pure ingredients and some, which make the body work hard to process into the active form as they are synthetic. Many people with ME simply can't transform them into the active ingredient and the vitamins will simply be pe'ed out and not processed. They are also often packed with fillers, which have no nutritional value. The only things I found worked for me that you can buy in the chemist was Floradix, the gluten free one is best as many ME people have a gluten intolerance - I think you may be able to get that one at Holland and Barrett but it's much cheaper online - I used to get it from Amazon. I don't use Floradix at the moment and use online retailers selling epigenetics and Nutri vitamins and supplements. If you google these, you will see the price difference as they are very high quality. Some supplements are the same whatever the price, such as D-Ribose but I expect you are probably wasting your money with the vitamins from the chemist.
I know I said you probably won't get far with the NHS but actually, there are a couple of thing I would try and get from the NHS. Firstly a blood test to determine her nutrient levels in particular B12 as you can get injections on the NHS for this if her levels are low. B12 is a very common deficiency in ME. The iron test may come back as OK as I think they will only test serum ferritin, which isn't a great indicator and doesn't necessarily mean she's not iron deficient. Also she is likely Zinc deficient - going into puberty many children are and they could also check for this. There is also something from the NHS, but it's still in the trial stage I didn't take that much notice as it only works if you've had ME less than 2 years - someone on Mumsnet is bound to know about it.
As I said, I see a trained toxicologist offering specialised detox therapy and she has helped me loads. She has been very honest and told me that I have a low grade infection, which she cannot get rid of and I need medical intervention. I still continue to see her because her treatments are a fundamental part of the process. I'm wondering if I was a bit quick to dismiss the osteopath as he may perhaps be a bit helpful while you save up to see a medical doctor. At the very least he can give you a bit of an insight into nutrition and vitamin supplements and as long as your mother is paying for this, it may be the first piece of the puzzle as you wait to have enough money to pay the thousands you will need for the medical doctor.
If you google the Breakspear clinic, you will also see blogs dissing the clinic as a hoax because one person's experience is very different from another's. So you will always find someone, who will say this osteopath is a wizard and others, who say he's a quack - the same as the Breakspear clinic. For example, if it's going to take you a year to have enough money, you've wasted a year where your daughter may detriorate of becomes bedridden. Besides, something the Brakespear clinic prescribes may not even work because as I said, our bodies all respond differently. I spent 5.5k plus travel and expenses and hotels on something, which didn't help me with the medical doctor because I originally couldn't take the medicine he wanted to give me. With the help of the toxicologist, I can take the medicine he originally prescribed, which costs 2k for just under 90 days.
If my mother offered to pay for anything, even if it wasn't the exact right thing, I'd be careful at what I took from the practitioner but I'd bite her arm off as treatments are very very expensive.