If I leave fellow NPCs (like Ian) somewhere, do they always return to their "original" location? Or where can I find them in that case? (I left Ian in the desert, but couldn't find him in Shady Sands afterwards, so I restored my save so as not to lose both him and the things in his inventory).
If I use a rope on the crater at The Glow entrance (so I can get inside), can I take it back or do I have to leave it there forever?
Thanks very much for your answers.

The NPC:s will stay where you leave them (if you leave them in the middle of nowhere I think they disappear, not sure though).
You can't get back the rope once you've used it. They're not that rare though, so it shouldn't be much of a problem.

NPCs remain where you leave them; make sure not to leave them outside of a green circled "static" zone on the map, or they'll be gone forever.
You can't get back ropes after you use them, but it doesn't matter that much because you don't use that many of them anyway and they're fairly common.

Thank you. IIRC, I could take the rope back after using it at Vault 15 and just found it weird not to be able to take it back at Glow.

where can you find the rope?

Try bartering with Seth (the one at the entrance) in Shady Sands.
Other places are listed for example here:
http://mikesrpgcenter.com/fallout/items.html
(but watch out for possible minor spoilers).