Hi, Jacquie,
I have walked the Lebaniego from San Vicente de la Barquera to the monastery at Liebana and then continued on the Vadiniense from Potes to Mansilla de las Mulas/Leon. The walk from Potes to Fuente De has a few roadside kms at the beginning but has some very nice kms. However, if I were going to spend a few days just walking in that area, I'd park myself at Fuente De, probably in the parador, and do some of the walks in the cirque just heading out the door and then from up high after riding the cable car. My blog from that year (in my signature below) might have some things to help out. The stage from Fuente De on the Vadiniense walks part of the Senda de Remona, and there are lots of connecting trails that would lead you back to Fuente De instead out of the mountains and down to Portilla de la Reina.
I remembered and found a WSJ article on the topic, hope it's not blocked to you.
http://www.wsj.com/articles/SB10001424052702303908804579566310546254896
As far as transportation goes, when I walked, there were several people who walked from San Vicente to Potes/Liebana and then wanted to head back to the Norte, and they took a bus to Unquera, so that's probably the best way, as you have suggested.
I know that several forum members have walked in the Cangas de Onis/Covadonga area also, but you might have better luck posting a thread with those words in the title to catch their attention.
Buen camino, Laurie