Ranthr - Ha! I too was curious as to why you wanted to bus it to Fuente De. Now we know!
I walked this stage three weeks ago. I was a bit dehydrated by then and a bit intimidated by Gronze's 5/5 difficulty rating. I'd been to Sto Toribio the day before so planned to go along the road for some of the way. And where I did, the road was absolutely fine - very quiet in the morning. I came off the road at Beares and got lost in the hills, so decided not to go off it again at S Pelayo. Walked on the road to Los Llanos and took the off-road path from there (which was a lovely path & track along the side of the hill) to Arenos/Cosgaya. There I met an English couple who'd come especially to walk the next off-road section which they knew. They tried to convince me to join them but as it's about 6k longer and I still had Gronze's scary rating in my mind, I stuck to the road. Which was a big mistake as I arrived in Espinama to my surprise just after 11.30 (I started at 6am). So it's very walkable and you have several places you can decide to take the road or the longer nicer paths.
I stayed in the mountain refuge in Fuente De, and enjoyed it, but if you want to cook you should bring food from the little supermercado in Espinama. When I was there the cable car cafe was closed, but some fellow pilgrims had the most amazing gigantic tortilla sandwich/loaf made for them at the Parador (for €6).
The route up from Fuente De to the highest point on the camino (with wonderful vistas) is all on a 4x4 track and very well marked, and only takes a couple of hours. So I'd say it's do-able for a seasoned camino walker at most times of year. You could then either follow the camino on to Portilla de la Reina, go back to Fuente De or go across to the staffed mountain hut at Jermoso (if it is open and the weather is good and you don't mind a little bit of mountain path at the end).
Sorry for writing so much - it's just it's still all fresh in the memory! I'm going to start a separate thread about what I found about Lebaniego/Vadiniense in the next week and post photos.
Cheers, tom