Okay, I'll address your points and add a correction.
This plant is intended as a deadly magical defence - the main sub that uses it is still under work, but it is supposed to be deadly and difficult to circumvent.
Magical winds, very fine dust masks, poison immunities, etc, are valid defences. Heavy rainfall would mitigate the deadliness as well.
Also, I was deliberately vague on how much pollen would actually cause death so it is easy to allow for lesser effects.
As to effects of Cardiac arrest, it would be equal to a non-magical death spell :)
However, I did make a mistake on this sub, so some magical alteration of the standard Digitalis is needed:
From Textbook of Materia Medica, 5th edition, 1932, page 259
Since Digitalis is slowly absorbed, and excreted still more slowly, it does not cause acute poisonous symptoms.
To achieve the desired level of lethality in line with my intent on this submission, one should instead substitute Cyanide, a much more acutely deadly toxin and one which does occur in many plants. Though I took the easy way out, and just made the toxin a variant of Digitalis. Go to Comment
A magical plant (Fungi!!!) that powerful has to have an interesting origin! Perhaps a murder victim spoke some appropriate curse on their killer as they died, and the fungus which sprung forth betrayed the murderer to his own death? Maybe? :) Go to Comment