Hi, interesting questions. My response is based on making the pilgrimage last fall. (It also seems to evolve into a glowing review of the author of my primary resource on this camino...
) Buen camino, wherever you walk, and in whichever direction!
"Ender"’s guide (Jose Antonio Cuñarro) states a Salvador credencial can be ordered online from the website of city of La Robla. I'm sorry but I've been unable to load the page on my laptop so I can't send the link. Try googling 'ayto la robla venta de credenciales" and you'll see the website. However, it seems you still may have to collect it in Leon...
If you're interested, the Salvadorana is the “compostela” of the Camino San Salvador and is issued by the cathedral in Oviedo as well as the Oviedo municipal albergue. The pilgrimage is from the Pulchra Leonina (León) to the Sancta Ovetensis (Oviedo), to pay respect to the holy relics housed there. Not 100% sure whether you’d be granted one by arriving on foot from a direction other than the Camino Salvador, but I suppose you could ask when you arrive. (BTW, mine does not look at all like the image in Ender's guide, unfortunately, but I treasure it anyway.)
I did not see anyone turned away from the albergues when I walked last fall, and I think I was the only one who presented the "official" Salvador credential each of those nights.
@peregrina2000 has done a terrific job editing and translating Ender's guide into English. I found it most helpful. Using it to go "backwards" might be challenging, but it's full of useful information and advice from the man who is instrumental in making this walk accessible to so many others.