Why に used with ヨリを戻す and [person]ができる?

Question

I have difficulty understanding the usage of に in two sentences I found from manga.

もう彼に彼女とヨリを戻す気は...

He no longer wants to get back to his girlfriend.

and

私には素敵な彼女ができた。

I got a wonderful girlfriend.

In the first sentence, why 彼に is used instead of 彼が? Is it similar to に used in passive sentences?

In the second sentence, why XにYができる is used to mean "X took up Y" in the context of relationships? I don't understand why this particular construction used. I prefer XがYとできる.

I already read this answer. It doesn't help much because if I try to translate the two sentences above literally with "by/unto" it will make things strange and awkward. For example, the first sentence can be understood literally as

Getting back to his girlfriend by him...

It is difficult to translate.

What is the role of に in the two sentences above?

Answer 1

First, understand the construct 彼に食べる気はない, 彼に帰る気はない etc. に is used to indicate you are talking about his attributes (e.g. 彼に足はない、彼に知恵はない and so on).

So, when you want to say he doesn't have the attribute 彼女とヨリを戻す気 anymore, you'd say 彼に彼女とヨリを戻す気はない

Answer 2

The answer is actually the same in both cases: the particle isn't が　because it's not a subject.

1. もう彼に彼女とヨリを戻す気は... [He no longer wants to get back to his girlfriend.]

First of all, as mentioned in the comment above, the full phrase is もう[彼に][彼女とヨリを戻す]気はない and the more literal translation is "He doesn't have the mood (気) to make up with the girlfriend (彼女とヨリを戻す). As all other　doesn't have sentences in Japanese it can be stated as XにYはない　(or ありません (I think, it could've also been XはYではありません)

2. 私には彼女ができた。 [I got a girlfriend.] Here as well, the subject is the girlfriend because she is the one that できた - "became", "appeared" whatever you like. To whom? To me. So literally: A girlfriend appeared to me. Or, with topicalization: As to me, a girlfriend appeared.