Interesting, thank you a lot for that analysis!
My notes says this about Datura metel seeds:
From herbarium in India: 1250 mg/kg scopolamine, 650 mg/kg hyoscyamine (Shah & Khanna, 1964)
Seeds from 2000, Poland: 756 mg/kg scopolamine, 914 mg/kg hyoscyamine (Mroczek, 2006)
Seeds from 2002, Poland: 425 mg/kg scopolamine, 1214 mg/kg hyoscyamine (Mroczek, 2006)
Var. fastuosa:
From herbarium in India: 3700 mg/kg scopolamine, 420 mg/kg hyoscyamine (Shah & Khanna, 1964)
From Poland, 773 mg/kg scopolamine, 1027 mg/kg hyoscyamine (Mroczek, 2006)
Unknown location, 76 % scopolamine, 24 % hyoscyamine (Bucher, 1989)
As it shows very clearly here, the alkaloid make-up seems to vary greatly. Note especially the great difference in the seeds from year 2000 and 2002, where you can see that the hyoscyamine content increases with age while scopolamine content drop, just as it is noted with other Daturas.
The general trend, however, seems to be that Datura metel seeds is scopolamine based. I wonder if you grew this plant yourself, or have any other way of determining the age of the plant the seeds originated from?