...はいい＝not needed ≠...がいい? Does ...はいい have no subject?


I am trying to ask a question with several linked open ends at once.

いい has the meaning that something is [not
needed](https://dictionary.goo.ne.jp/word/%E3%81%84%E3%81%84/#jn-9174). It
seems to be related to the distinction of は vs が. For example here is a
dialogue (from the film 海街diary) that I had been puzzled by at first.

> すず：私、手伝います。  
>  幸：すずはいいから、荷物の整理しなさい…

(The context is that the character すず moves to live with 幸. Having just
arrived with lots of luggage, すず offers to help 幸 prepare lunch.) At first I
thought "すずはいい" meant "Suzu, you are a good kid" or "it is nice of you to
offer to help" but that was not what the English subtitle said. いい here should
mean (that you help is) not needed.

This [answer](https://japanese.stackexchange.com/a/73186/38770) to a related
post reminded me of the dialogue. Listed below are some of my thoughts and
observations. Are they correct?

  1. In "すずはいい", すず is the topic, but the topic here definitely does not coincide with the subject. It even seems that いい does not have a subject at all--the predicate いい seems [avalent](https://en.wikipedia.org/wiki/Valency_\(linguistics\)).
  2. Therefore, if we change は to が, then すずがいい has to mean that Suzu is good/nice/kind...
  3. By this logic, if we want to use いい in the sense that something is not needed, we cannot say ...がいい. It would have to be ...はいい. Example sentences in dictionaries seem to support this claim.
  4. As a side note, it happens that the sentence "僕は良いよ，ご飯食べたばかりだから" in the context of turning down an invitation to have a meal together can be translated as "no I'm good..." in American English. Surely that is just a coincidence?

