Hi, Luka,
Based on what you say, I don't think the Invierno is what you're looking for, because you really have to be prepared to be alone all the time. I was able to reduce the isolation because I speak good Spanish and found that the people in all of these places were friendly and chatty, and by taking a 2 day break at ribeirasacra.com with a forum member who has a gorgeous casa rural (B&B) about a km off the Invierno on a hill overlooking the Minho River. Yes, I wrote the online guide, but I have gotten lots of help from those few others who have also walked the Invierno.
The Salvador's traffic has increased a lot, but again it's very likely that you won't run into anyone. The hospitalera in Pajares told me that the numbers had doubled or tripled in the last year, but since they were starting from fewer than 100, this increase doesn't mean you're guaranteed to have any company at all. As I said, I was completely alone, but it is so beautiful and is a nice prelude to the Primitivo. It's 120 kms, with a day with some hefty elevation gain, but it's easy to break the walk into 5 or even 6 days if you want. Ender has written a great guide as well, which is also translated into English
http://caminodelsalvador.net/?cat=13 (Ender is the person most responsible for making the Salvador accessible to pilgrims -- he has put lots of markings on the remote mountain day to ensure you are never far from an arrow or conch shell).
The Primitivo may be just what you're looking for. It has pilgrim traffic and lots of wonderful albergues. If you look through the threads, you can see stories of many positive experiences, and there are suggestions on how to break it into shorter stages than the usual ones (see particularly Tio Tel and Tia Valeria's postings, they have been on this route several times and give great information). The only caveat I would give here is that from what I've heard, the Primitivo in August becomes clogged and overrun. I know several Spaniards who started out in August and left. The infrastructure can handle a stream of maybe 30 a day, but one day last August there were well over 100 people in Cadavo, for instance. I went in mid-late June and had absolutely no trouble. The only day we had a crowding problem was in Berducedo, where the municipal albergue has 10 beds. But there is a private place that's very nice and easily absorbed the overflow.
One last question -- what time of year are you planning to walk? Winter on any of these could be somewhat problematic. Buen camino, Laurie