Create my own addresses, roads and ways on a custom world map

I need to render a full map, and I also need some kind of API to generate markers and roads easily.

The project

It's for a specific RPG. This is a game with its own world map, its own cities, frontiers and roads. So a full map, which I can create easily any data to import in any database, xml, json, kml... ANYTHING ! :)
So I'd like to use any API to make a map for this game.

What I need

In fact, I'd like to create a whole new map, completely out of our world map.
I have a full JPG map which I have to cut into tiles, then I need to navigate through all of the tiles with the ability to zoom and to change the tileset depending of the zoom value.
So, I need to import datas about towns, cities, frontiers and roads.
Then, I need a system that can calculates the way between 2 cities by searching the available roads between the points.
I know it can be hard because it's almost making another full Google Map but for another "world", and I'd really like to make it.

My questions

I don't want to use GoogleMaps (just because they change their API too often, and it's not open-source), so I try to find infos about OpenStreetMap for example.
Do I have to develop a whole "Geocoder" (cities, roads, itineraries...) server ? For sure it's gonna be less weight than Google, because this RPG's world is not bigger than France or England, and it's an RPG so there is not as much details as in our real world, but if it's really needed to make an API, let's do it then !

It would be such a real pleasure for you all to help me, or at least to guide me for all the "how-to's", cuz if this project comes to birth, it would be such an amazing unique work in any RPG game !

I heard about Maperitive, Mapnik or Leaflet, but they all seem to work with OSM datas, which I don't know yet how it works. Moreover, they do not seem to be able to generate map with a jpeg tileset (whereas google does, but I don't want to use Google, I need an open-source software), they render maps from some kind of xml datas. I can use them, but then I'll have to use the tileset in other ways (background or something like that). It's feasible, but it's not the problem.
The problem, actually, is that I just need to use my JPG map, use and generate datas about cities, roads and anything relating to maps, and further, an itinerary calculation system (which I think I'm gonna have to develop, as I cannot find any solution on the internet).

Sorry for the huge block text, thanks a lot to the ones answering me ! :)

Greetings