I'm confused by take their spot.

There is no holding of spots it is open to everyone. A GM made that clear a couple times, however for level 45+ ones it is group based, a single person shouldn't be out there starting them or holdign the npc.

No one group has more right over another to start caravans. Not sure if they changed it ut have seen two runnign at a time in certain spots.

However courtasy is important, there are other spots and typially if a group is in one, another group will go to another location.