Are you allowed by copyright to do that?
“Are you allowed by copyright to do that?”
Yes.
Unless NN asks not to. You are correct it is their copyright and they have the right to ask a web site not to post their material, even for non-profit educational purposes.
Longer excerpt is ok as well unless the source specifically asks for no excerpt at all.
Theoretically excerpting is always fair under fair use, but a source can ask for no excerpt or anything at all.
A web site such as this could probably sue for use of fair use excerpts but it wouldn’t be worth it.