Yes, unless you're in a pro race with a caravan. It'd be much too dangerous for people to just randomly take feeds.
No.
Yes, anyone that's handing something up can hand it to you...
Of course.
Each individual race will have feedzone designations. Some races have neutral that anyone can have. They'll usually let you know before the race, though, and neutral will shout out something like "neutral" or "water" so you know which one to grab. It's quite frowned upon to steal someone else's bottle, but I see it happening from time to time when guys get desperate. Can be fairly comical if the owner of said bottle sees the transgression and confronts the "thief".
