ことにする/なる and ことに決める

Question

Reading this answer that said that ことにする is an idiom, and I began to wonder what the origins of this idiom were. I believe that since the two grammar structures are related, ことになる would also be an idiom if that is the case.

Additionally, it crossed my mind that ことにする and ことに決める have similar usages and meanings, but I haven't heard a conjugation of 決める that would have a similar meaning to ことになる。 If I had to guess, you might have to either use the intransitive form, 決まる、or you might want to say that someone decided that (verb).

So how would I rephrase this sentence using 決める or its intransitive counterpart?

私は転勤｛てんきん｝することになりました。

Would it be this?

私は転勤することが決まりました。

So to sum up my questions shortly:

1) What are the origins for the idioms ことにする and ことになる？

2) Can I replace the ことになる grammar with the 決める verb or its intransitive counterpart?

Answer 1

This might be very incorrect, but I thought I'd try and answer from my limited knowledge and my form of understanding how this works:

1) Not sure about the origins of the idiom, but you might gain an understanding from looking into the direct meaning of each phrasing:
- ～ことにする >> "in the direction of doing (the thing)"
- ～ことになる >> "in the direction of becoming (the thing)"

Expanding on this idea:

私は今日どこかに出ることにする。
I have decided to go out somewhere today.

In another way of phrasing it, "I am in the direction of doing the going out to somewhere today." This sounds nonsensical, but if you frame it in the idea that you are "in the direction of", or "leaning towards", rather, doing that thing, then it makes more sense, right?

Likewise for ～ことになる:

牛乳を長い間に飲まないと、腐ったことになる。
If you don't drink the milk for a long time, it will become spoiled.

Or, "If you don't drink the milk for a long time, it is in the direction of becoming something that's spoiled."

2) I don't think that using ～ことに決める to replace ～ことになる can be used as directly as I think your question is asking. ～ことにする implies that something is leaning towards a specific course of action, which could also be expressed by a decision, which is the direct meaning of the word 決める "to decide". However, leaning towards a change of state can't be expressed by 決める, there a different verb applies, such as 変わる.

Tl;dr -- the clue for understanding what's going on here grammatically speaking is in the particle に and its usage. The rest should be taken with a more literal interpretation.