Some scattered thoughts raised by your questions. Walking “backwards” on any camino is always a challenge, because it is only marked in one direction. And it won’t be like the Francés, where you can just keep walking towards the crowds coming at you. But it would be very easy to do with a GPS. I don’t know where you have seen that many Norte pilgrims skip Gijón by going down to Oviedo and then back up. I have done that several times, but have never met more than one or two people on the way “up” to Avilés from Oviedo. But the route is very well marked and not industrial, so it has the advantage that you will skip the industrial entrance into Avilés you would have on the the Norte.
Walking from Villaviciosa to Oviedo has the additional advantage that you can take a very short detour to visit an amazing pre-romanesque church in Valdediós. Even if you don’t like ancient religious architecture, the setting is spectacular, church and monastery nestled in a valley. The way up out of the valley is short and stiff, with great viEws back down. It is only about two km longer than the more direct route— you can see a schematic map here.
https://www.gronze.com/etapa/villaviciosa/pola-siero. The last time I did this route, I left from Villaviciosa early in the morning, had to wait a while for the church in ValdediĂłs to open, and then had a long picnic in the meadow just soaking it all in. So I decided to stop in the albergue in Vega de Sariego, 10 kms earlier than I had planned, which was perfectly adequate. I have also walked on further to Pola de Siero, and its albergue is superior. Pola is a much bigger town, an Oviedo suburb in many ways. If you are going to want to spend time in Oviedo, of course, it makes more sense to walk on to Pola de Siero so that the next day you have just a very short walk and can spend most of the day enjoying Oviedo.
Avilés has “cleaned up” its historical core a lot. I was on the Norte there a few years ago with a Spaniard who was pretty dumbfounded to see all the tapas bars, pedestrianized streets, etc. It is pleasant, but I agree with
@oursonpolaire that there is not much to see there. Not like Oviedo, which is a beautiful city with lots to see and do.
The walk from Pola de Siero into Oviedo is your typical commercial outskirts of a city — lots of car dealerships, but no industry like you would get walking into Avilés. So that makes it, IMO, more tolerable, though it isn’t beautiful by any means.
Buen camino, Laurie
p.s. And though this isn’t anything you have asked about, since you are going to enjoy a lot of flexibility, you might want to consider taking some of the many absolutely beautiful coastal options that are oh so close to the Camino del Norte, which frequently trudges along the side of the highway. Check them out here:
https://www.caminodesantiago.me/community/threads/coastal-alternatives-to-the-nortes-asphalt.49578/