“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.
ok makes sense