Hello Spunkier,
That is quite a battery of tests. It is difficult to make the diagnosis without a thorough history and physical plus the tests you mentioned. I imagine you are also heading for an upright tilt table test as well.
In the end my guess is that you have orthostatic hypotension (ie when you stand up, your blood pressure enough to cause symptoms). I do not think you need to get too worked up over the long term consequences of this, although I recognize it is having a significant impact on your daily life now.
The echo will ensure that you have normal heart function -- and I will be bet that you do. I would also check a holter monitor to make sure there are no arrhythmias being missed, but again, this sounds pretty classic for orthostatic hypotension.
Orthostatic hypotension or in its extreme form postural orthostatic tachycardia syndrome (POTS) is difficult to treat and involves increasing fluid and salt intake as well as some behavioral changes like not standing up too quickly. If you end up with this diagnosis, it is important to make sure you control it and not it controlling you. Learn your limitation and realize this disease and these symptoms will not kill you -- I say this because many people with diagnosis continue to worry that their real diagnosis is missed and are convinced they are dying.
This a bit long winded on my behalf and probably premature since you don't have a diagnosis yet, but these are issues I wanted you to be aware of.
Good luck and I hope this helps.