Not 100% sure about the leader question, but to the best of my knowledge I believe they have to be present in the settlement they are assigned to lead.
Caravans, as designed in SS2, and provisioners are functionally the same on surface as both connect the resources of settlements together. Some primary differences are;
1) Caravans are automatically assigned based on a) proximity and b) if the target settlement also has a caravan plot. Provisioners however can be assigned without either of these conditions as long as you have the first Local Leader perk.
2) Provisioners can be attacked and so a supply chain can be interrupted temporarily, meaning you lose access to some, or all, of your other settlement resources. Caravans don't suffer from this BUT they only update available supply once per day if I am understanding the mechanic right.
So both have bonuses and drawbacks. I would say no you don't need both, but really it's up to you how you wan to assign your settlers.
**Edit
Also bear in mind that the beacons you craft at the planners desk only work with caravan and communication plots active, and the range is limited (ie. Sanctuary having both could send people to pick up stuff at Station Olivia and Concord, but not loot left in Lexington. just as an example but not may not be accurate)
