LC Doc, regarding the teenager - seems to me like you describe POTS - (Postural Orthostatic Tachycardia Syndrome)?
I have a little personal experience with this in a relative - who had low BP in the mornings.
Once that was solved, the POTS corrected itself.

However, if BP normal...(note that nadolol has major side-effects, drug-drug interactions, and a black-box warning...)

[link to petermcculloughmd.substack.com (secure)]
"Most of the care for POTS is supportive (lower leg compression stockings, increased salt and fluid intake, exercise). I have found in severe cases with adequate blood pressure that the use of nadolol can control the tachycardia and help while the patient has cardiovascular-neurological reflexes return to normal over a period of months."