As soon as I read your post it struck me that all your dd’s symptoms could be caused by or attributed to Postural Orthostatic Tachycardia Syndrome (POTS) which is not rare - as the newspapers regularly report it is, but actually quite common, especially in girls of your dd’s age, but just underdiagnosed.
Are the faints positional? As in, does she faint when standing or having moved from lying to standing, lying to sitting or sitting to standing? If it is POTS, the panic comes from the adrenaline surging around the body as a result of the heart working extra hard to try and get blood up to the brain. When the blood doesn’t get there, that’s when you get the faint.
If you are able, check her pulse after lying quietly for 10 minutes, then ask her to stand and check it again at 2, 5 and 10 minutes. If it rises by 40 beats or more and stays up, it could be POTS and I would take the info from the POTS UK website to your GP and ask for a referral for further investigations. That would usually be cardiology initially to rule out any cardiac issues - but most cardiologists know very little about POTS itself. Dependent on your hospital system it could be Neurology, Neurophysiology, Electrophysiology or a Syncope Clinic (often in Geriatrics!) for the actual diagnosis.
It is horrible and affects quality of life, but isn’t dangerous and can be managed by staying well hydrated (3L of water a day minimum) electrolytes (eg Nuuns hydration tabs) compression stockings and in some cases salt loading (although this shouldn’t be done unless medically advised). Exercise to strengthen legs (exercise bike, rower, walking and resistance training) is also important as it increases the muscular-skeletal pump, which helps get blood up to the brain to prevent the faints.
If the above conservative measures don’t help, there are then medications they can try to increase blood volume and/or slow the heart rate to reduce the tachycardia.
Both my son and I have POTS. I have had it since I was 7, but was only diagnosed in my mid-late 40s. The majority of cases start in puberty and have improved enough to no longer meet the diagnostic criteria by early 20s. When the symptoms are being well managed it’s not a massive problem, but it can take a bit of tweaking to get the balance right for each individual.