My research was done by scouring the web some time ago. No links but here's what I got.
1. Primitivo is indeed the first Camino route and King Alfonso II from Oviedo is technically the first pilgrim.
2. It was the worst time as most of Spain was under Moorish rule with the Christian kingdom being pushed up north towards the mountains and the coast.
3. As the news of the apostle's tomb spread outside Spain many pilgrims started coming in from France, etc. The Norte (Coastal Route) is probably what they use, making it the second route before the
Camino Frances. Not much was documented on the pilgrims walking the Coastal Route but it seems that the pilgrim will walk along the coast and then head south towards Oviedo and continued to Santiago from there.
4. When the
Camino Frances came about, it becomes the main route since Coastal Route is really tough. Numerous headlands and rivers that couldn't be crossed unless one walks miles inland until the river flow is small enough or if they are lucky to find a bridge (imagine a 20 meters rivers crossing may mean a 5 km x 2 distance instead)
4. As the reconquest started and most of the land around
Camino Frances are taken back from the Moors, it became clear that walking inland is much easier and so the
Camino Frances slowly started to take shape. There was a primitivo
Camino Frances route where the pilgrim cross over to Spain from the coast (Irun) and then made it via the inland route from Irun to Santo Domingo de la Calzada and then continued westward. When the current mountain pass from St. Jean became an option it quickly became the preferred route of the pilgrims outside Spain.
5. During the medieval time, there are not such thing as to what is the actual way. Pilgrims just walk and get to their destination based on the situation and directions from the locals. And for religious reasons it is also not uncommon for pilgrims to head north after Leon to Oviedo (Camino San Salvador) and obviously increasing the distance to see the relics of Jesus before continuing their journey to Santiago.
Camino Frances was first documented by Frenchmen Aimery Picaud who wrote probably the first lonely planet guide. The route he took and documented became the basis for the current Camino France route.
6. This route attracted many French and many even stayed behind (probably to open up business along the route much like today's camino industry) and so it is aptly named the French route.