I have twice walked the route on the southern side and found it well-marked. You will have no trouble. The northern side is less well-marked (well, it really isn't at all) and I found myself in the middle of a field with a shepherd showing me the GPS and google map in his IPhone (only on the Camino!!!), but it has the reward of the hot springs at Tiermas at Km 336--- there is much walking along the side of the carretera and only one place for water, so it is not agreeable. Unless you are a hot springs fan, stick to the southern route through Ruesta.
Sanguesa to Javier is about 10km and a further 8-9km to Leyre. The way is not marked, and while most of it is along the carretera, google maps shows a way from Javier to Sanguesa which avoids the N-5410-- it is parallel to it, to the north, and leads from the castle at Javier. While I did not follow it, I saw it from the road and saw that it was easily done. In any case, the carretera (N-5410) is not that busy.
I have no information or experience on Lumbier.