有無を言わさず・有無を言わさぬ

Question

I understand "対立する言葉", in this case 有無, however, I cannot yet grasp these expressions:

有無を言わさず, 有無を言わせぬ, 言わさぬ.

The definition according to Weblio is: 相手の好むと好まざるとにかかわらず、物事を強いるさま。承知不承知を度外視して行わせるさま。「有無を言わせず」とも表記する。相手の諾否に関係なく。無理やり。有無を言わせず。

有無を言わさず、宣告を下す。
== He/She was forcibly sentenced (no questions asked)

If I understood correctly, this sentence seems to agree with the definition.

However, some other sentences like:

有無を言わさぬ迫力があった。
有無を言わさぬ口調だった。
有無を言わさぬ物言いだった。

...seem to indicate that 有無を言わさぬ means: unspeakable; indescribable as in "too much".

1. Are my translations correct?
2. Is 有無を言わさず identical to 有無を言わさぬ?
3. Can you provide a few example sentences using 有無を言わさず・有無を言わさぬ

Answer 1

ず is the continuative form (連用形) of the negative auxiliary ぬ. It follows the pre-nai/irrealis form (未然形). In other words, ず and ぬ are different conjugations of exactly the same word. (Note that when ぬ follows a continuative form, it's a different auxiliary for perfective aspect that conjugates differently.)

The difference between ぬ and ず is simple; ず modifies something adverbially, whereas ぬ modifies something attributively (aka adjectivally) or works as a predicate. So 有無を言わせず is forcibly and 有無を言わせぬ is forcible, undoubtable or indescribable depending on the context (literally, "(which) doesn't let [someone] say yes or no").

言わせる (ichidan) and 言わす (godan) are two causative forms of 言う. They mean the same thing, although 言わせる sounds more common in ordinary speech.

So we have the following combinations:

	連用 (adverbial)	連体 (attributive)	終止 (predicative)
言わす	有無を言わさず(に)働かせる 有無を言わさないで働かせる	有無を言わさぬ迫力 有無を言わさない迫力	彼は有無を言わさぬ。 彼は有無を言わさない。
言わせる	有無を言わせず(に)働かせる 有無を言わせないで働かせる	有無を言わせぬ迫力 有無を言わせない迫力	彼は有無を言わせぬ。 彼は有無を言わせない。

Related: What is the difference between the negative forms -ず and -ぬ?