You may be able to save someone's Instagram story video, but it's not always straightforward. Instagram has its own policies, and if the owner of the story has chosen not to allow saving, you won't be able to. Plus, be cautious of using untrusted methods as they could lead to account issues.
Yes, you can, but it depends on the settings and privacy options of the person who posted the story.
In some cases, you might be able to save someone's IG story video. However, if the person has restricted saving or if it's against Instagram's terms of use, then you can't. Also, there are third-party apps that claim to do this, but using them might violate Instagram's rules.