I know new users (<50 rep) can comment on their own questions/answers. Can they also comment on others' answers to their questions?
If not, that should be allowed as well.
Yes. This has been around for a long time, but you would never know it by how many people continually ask for the already implemented feature on UV.
I updated the /faq on all sites to reflect this, because it was.. er.. a frequently asked question.
Yes, they can. I've seen it done many times.