Usage of 次第 with informal language (やる)


I encountered the following multiple-choice question on page 29 (question 10)
of my JLPT N2 grammar textbook 「新完全マスター文法 日本語能力試験Ｎ２」:

> 佐藤君、今やっている作業が（ ）、山口君の方を手伝ってやってくれ。
>
> １ 終われば ２ 終わった上で
>
> ３ 終わり次第 ４ 終わったかと思えば

I chose 「１ 終われば」, but the answer given was 「３ 終わり次第」.

My textbook states on page 17 that ～次第 in the sense of 「～が実現した後、すぐに続けてある行動をする」
is a 「硬い言い方」 (formal/stiff expression), which does not seem to go well with
the informal tone of やってくれ here, so I did not choose 「３ 終わり次第」. 「１ 終われば」, on
the other hand, seems to make sense to me:

> （？）佐藤君、今やっている作業が終われば、山口君の方を手伝ってやってくれ。
>
> Satō, help Yamaguchi if you finish the work you're doing right now.

Why is the answer 「３ 終わり次第」 instead of 「１ 終われば」? What am I missing here?




I guess 今やっている作業が終われば is acceptable, but the whole sentence with 終わり次第 is more
natural as an order in office.

Yet another possibility is use of たら. To me,

  * 今やっている作業が終わったら、山口君の方を手伝ってやってくれ。

is completely natural whereas 終われば is very slightly unnatural. The difference
is subtle, but 終われば sounds like _in case/if ever you finish the job_ where the
possibility of finishing the job is low. On the other hand, 終わったら sounds more
simply _if/when you finish the job_.

Because of this slight unnaturalness, 終わり次第 = _as soon as you finish_ is the
(better) answer.

* * *

There are a number of websites explaining conditional expressions
([1](https://nihongokyoiku-shiken.com/to-ba-tara-nara-in-japanese/),
[2](https://sensee.jp/media/teaching/difference-to-tara-ba-minnano-nihongo/),
[3](https://tanosuke.com/to-ba-tara-nara)). In 1, it is stated that ば is used
for hypothetical condition and たら for a one-shot event, which I think explains
the above to some extent.




I guess 今やっている作業が終われば is acceptable, but the whole sentence with 終わり次第 is more
natural as an order in office.

Yet another possibility is use of たら. To me,

  * 今やっている作業が終わったら、山口君の方を手伝ってやってくれ。

is completely natural whereas 終われば is very slightly unnatural. The difference
is subtle, but 終われば sounds like _in case/if ever you finish the job_ where the
possibility of finishing the job is low. On the other hand, 終わったら sounds more
simply _if/when you finish the job_.

Because of this slight unnaturalness, 終わり次第 = _as soon as you finish_ is the
(better) answer.

* * *

There are a number of websites explaining conditional expressions
([1](https://nihongokyoiku-shiken.com/to-ba-tara-nara-in-japanese/),
[2](https://sensee.jp/media/teaching/difference-to-tara-ba-minnano-nihongo/),
[3](https://tanosuke.com/to-ba-tara-nara)). In 1, it is stated that ば is used
for hypothetical condition and たら for a one-shot event, which I think explains
the above to some extent.

