We have just returned from the Norte and Covadonga. The routes through the Picos are, in our opinion now, for mountaineers and mountain goats. The paths cross rivers by stepping stones; cross high steep ground; have some steep drops........ie - they are not for the usual style of Camino walking, not even if you have walked the Primitivo. The Guardia Civil, in Cangas de Onis, approved our decision to walk the Camino Real from Arriondas to Covadonga rather than trying to use the GR route from Llanes to Covadonga, or the GR from Cangas de Onis which is part of the Oviedo route. What looks good on paper might not work safely in practise.
We'll be interested to see what you finally decide and how you get on. Covadonga is well worth the detour, however achieved. It was at one time part of the official route from the Norte, through Covadonga, Oviedo and then onto the Primitivo to Santiago.