Donovan and Elsa seemed nonchalant about meeting a 700-year-old knight

Depends. I would have been thrilled to meet him sure, but Donavan was only interested in the immortality the grail promised.

And while I don't disagree with the premise of your question, to film it would be rather odd. Imagine Donavan keeping his gun pointed at Indy but excitedly chatting up the knight. That would be an odd distraction in that scene and would disrupt the flow of the movie.

Donovan was singularly focused on the Grail. While it would be interesting meeting someone who spent 700 years in a cave, the Knight was ultimately irrelevant toward that goal. He would not pose any threat to physically stop someone from taking the Grail, as we saw with Indy. And the only thing he could do at this point to fulfill his oath would be to not tell you which one it is. Even if you threatened to kill him, he wouldn't tell, and just consider dying as the last part of his duty to protect the grail.

I guess they were more focused on the chalice. It would be interesting, but also it wouldn't. The man presumably sat in that same room all that time, so it's not like he'd have any stories of witnessing historic events to relay

Good observation. Its a plot hole I guess

They were expecting to find a cup that promised immortality. Seems if they accept the possibility that it exists then could probably accept that someone might be using it.

Like has been said they were more focused on getting the grail. Also they were both too scared to try and get through the obsticals themselves. So of course they'd act rude toward the Knight. They also didn't bother thanking Indy for stopping the obsticals for them.