grammar - What verb form is くださねばなるまい

The following is from a mystery manga. The criminal mastermind is lamenting that his accomplice Masumi failed to properly kill Kimisawa.

ますみの奴・・・
最後の最後でしくじったな
あの至近距離でよもや急所をはずす

君沢ユリエのケガの具合によっては (new bubble if relevant)
また私が直接手をくださねばなるまい

Basic idea I hope: "That damn Masumi messed up right at the last minute. She missed a vital point from point blank range. Depending on Kimisawa Yurie's condition, ..."

I am having trouble understanding the last sentence, particularly the bold part. I have heard of the negative volitional form and assume that なるまい means the speaker has a strong will for something to not occur. This understanding however does not seem to be consistent with the によっては part. It seems to me that an action should follow and not a "will that a state not occur".

I am also uncertain about what form くださねば actually is. I am familiar with くだせば, くださなければ, くださせば but not the given form.

I also wonder why the speaker used はずす rather than はずした. The action of missing the vital point is clearly completed and in the past so why treat it as though it is not?

Answer

くださねば is くださなければ written in 文語 (classical Japanese).

私が直接手をくださねばなるまい。
I would not help doing it myself.

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よもや is a negative polarity item (i.e., expects a negative predicate after it). From 明鏡国語辞典:

よも‐や
副 《多く打ち消しを伴って》 ほとんどありえないという気持ちを表す。まさか。いくらなんでも。万が一にも。「━来ないということはあるまい」

So we can tell a main predicate is omitted after はずす, and the tense of はずす is relative to the omitted verb. The "full" sentence would be something like あの至近距離でよもや急所をはずすとは思わなかった or よもや急所をはずすとは信じられない. I hope these はずす now look natural to you.