To the people who are also interested in this "disambiguation"...
While I am still not a 100% sure I think I can now explain why in:
「大抵は魔法使いの エキスパートで―…」「大抵」was translated as "In general..."
If we consider the defnition:
Especially 「だいたいのようす」, after cheking the definition of 「だいたい」 again:
It seemed to me that the meaning of "larger/largest part of" only exists in combination with "specific entities" (as in 「大丈夫だって。大抵の奴はわからんよ。」)
In cases like 1) it seems to me as if 「大抵」 refers to "the elements that are shared by the main part and therefore construct the "usual case", or in other words what is the "usual/general state". In English you can simply use "mainly" or "generally" for this kind of concept, therefore the transaltion in 1) is perfectly fitting.
Extending on the definition in the first link:
is listed as well.
However it still seemed weird to me that once the word refers to "the usual state/case/etc" and is in another case used to refer to "the main part".
Looking up the definition in https://jisho.org/search/taitei did not really help either as they mark it as "mainly"(noun/na-adjective) and "almost all" (noun/no-adjective) and in 1) it remains underterminable whether it is an na/no-adjective.
In any case this seems to be a problem of language interference in a broader sense.
All definitions seem to share a common idea of something being "main(ly)/general(ly)"; however the definitions do not state that 「大抵」 refers to the larger part of something but rather to the "general"(in once an adverbial sense and once possible adjective sense in its status as na/no-adjective). Here the language interference comes into play, there is no such adverb as "general" in English, we need to use "generally"; however, it is impossible to speak of "the generally part*". Therefore, I think that my non-japanese brain unconsciously solved this problem by adding "part of..." to the definition of 「大抵」 even in its adverbial sense.
Please feel free to comment whether I am correct or not and even further if experienced somewhat similar cases with the use of adverbial nouns as well.