Morten St George Theory

[Morten St. George]

The supposed sunflower is most likely a cereal crop like millet. 

I completely agree with you on that, but rather than millet, it would almost certainly have to be corn, the staple crop of Mexico back then. I think the VMS artists simply decided to replace a head of corn with the interior head of sunflower.

Let's check it out. The drawing of the left is our VMS sunflower and the drawing on the right comes from a 16th-century herbal:



Notice that on the left side there are seven curved leaves in each, modest roots in both, essentially only the head is replaced. To confirm that the herbal does in fact depict corn, let's look at the complete page:



Confirmed: you see the heads of corn just below the plant.

And finally, from the title page of that herbal, let's look at what the author of the VMS marginalia is holding in his left hand:



That's right: a head of corn!