the main problem is that the arrows in Galicia only point in one direction, so may be ambiguous or not there if you're going the other way. In Portugal, the routes are supposed to be marked both ways: yellow to Santiago, blue to Fatima, but it's not too clear at the moment (at least, to me ;-) ) what the status is on this. The marking was supposed to have been finished in August, but there's still been no official announcement.
On albergues, the 'rule' is supposed to be that you need a credencial to stop in the albergues; end of rule. It shouldn't matter which direction you are going, or how many stamps if any you have in it. I seem to remember someone telling me they used a fresh credencial for the way back, but that's not compulsory. If you use a different route, you may want a fresh one to get more room for stamps. In Portugal, you can simply say you're going to Fatima.