Thanks Huda, I like this type of analytical thinking
About your first inquiry
The main thing responsible for hardness of nux vomica seed is the hemicellulosic endosperm which is not present in black mustard
, Presence of sclerenchymatous tissue in the epidermal layer and its prolongation into appressed hairs act as primary defense barrier for the nux vomica seed (so testa here is one integument) while in black mustard, the epidermal and hypodermal layers are weak tissues (parenchyma) so there is a sclerids layer to give the protection and defence to the seed (here the seed is 2 integuments).
Also, black mustard seed is very small (1mm diameter) comparing to nux vomica (2cm diameter) so no comparison in hardness between both
about 2nd inquiry
No, if you say that
Strychnine is very toxic in very high doses
whatever the rest of the statement is, you mean that strychnine is not toxic or has low toxicity in low doses
but actually, it is highly toxic in very low doses as 23mg) of strychnine may cause death,, and that is relative to other toxic substances and other drugs by comparing LC50
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