Alison,
I like the iMagT or the NeuroMag T from New Beginnings. However, there are other companies carrying the Magnesium L-Threonate which is good. With the iMagT I start with 1/2 teaspoon twice daily. If they are sensitive to supplements than a 1/4 teaspoon twice daily is a good starting dose.
The Low HVA and 5-HIAA could definitely be a need for BH4. The phenyllactic and phenylpyruvic are rarely elevated, but the benefit from BH4 supplementation anyway is still helpful.
Sincerely,
Dr. Woeller