The second opening for 四月は君の嘘 starts with these two lines:
忘れようとすることで傷が癒えないのは
忘れようとすることで思い出されるから。
I get the feeling that something is missing after から, like だ or です. Is this sentence still grammatically valid? Also, are the reasons for this omission as I suspect them that ending a sentence in plain だ is unusual and that a formal です would be out of place in such a song?
Yes, だ/です at the end of a sentence is often omitted. Do you know a rhetoric device called 体言止め ("noun-stop")? The omission of だ after から has the same basic effect (making the sentence compact yet vivid and dramatic).
This AのはBから(だ) is a cleft sentence formed from BからA. ("A because B" → "It is because B that A")
忘れようとすることで傷が癒えないのは忘れようとすることで思い出されるから(だ)。
Trying to forget doesn't heal wounds because trying to forget reminds us of them.
