What is a "final song"?
It's a tough question, but since it's said before もう二度と会えない, perhaps it implies "This is my last message/song to you".
Why is "nita" past tense and not non-past "niru"? Does this song no longer feel like a cradle? When did it then?
See:
- "太ってる猫" vs "太った猫"
- What are the general principles of using verbs to modify nouns (e.g. 焦げるトースト/焦げたトースト)?
- Use of かける (N5 question)
L. 2 can be read in like 4 ways:
Go with the first interpretation.
