The following is from a mystery manga. The criminal mastermind is lamenting that his accomplice Masumi failed to properly kill Kimisawa.
君沢ユリエのケガの具合によっては (new bubble if relevant)
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?