The teshuva to R’ Weinfeld proves that this is the case. The fact that it wasn’t printed in Igros is entirely irrelevant.
No. It was written in 1956. That could easily make it into the Igros. There is something a little bit strange about why it is not in the Igros. I’m not saying it is completely immaterial, but there is a pretty good question about why it is not in the Igros.