I don't see that anyone has bothered to contact the CDC,Center for Disease Control on Ergot,or the Department of Agriculture.
I have a friend with the Department of Agriculture,finishing up his doctorate.
His opinion is Ergot is a definite possibility and not so easily dismissed.Food preparation and
storage in the middle ages leaving a lot to be desired.
As for not having all the symptons,you can have a disease,for example Cancer,and not have all of the symptons manifest themselves,but still have Cancer.
The bottom line is no one knowns for sure what happened and why,and we may never know.
All possibilities are on the table till proven otherwise. Keep an open mind.