Hi, Ahaj,
I think it's a great idea to plan in four extra days at the end -- I always do that, and sometimes wind up using them on my Camino, but sometimes I find I've got the days to make it out to Finisterre and/or Muxia.
I also think it's not a bad idea for you to wait to see how you feel when you arrive in Santiago. You may be ready to hop on a bus or you may just want to keep walking for as long as possible. You can walk to either Muxia or Finisterre in three days, with one of those days being in the low 30s. You could then take a bus back to Santiago on Day 4. If you were really in crazy walking mode, you could walk Santiago-Muxia-Finisterre, and arrive in Finisterre in time to hop on a bus back to Santiago on Day 4 (many more buses from Finisterre to Santiago than from Muxia). I have done that, in a pinch, but of course it isn't as nice as being able to spend the night in Finisterre, go up to the lighthouse, etc.
As far as your specific questions, the only section of this camino that is well marked in both directions is between Finisterre and Muxia. I know there are more and more people walking back to Santiago from Finisterre, so maybe it's not as difficult as I imagine. But I think it would be a bit of a challenge walking from Muxia to Olveiroa. The walk from Muxia to Finisterre is pleasant, but very little is on the coast. To remedy that deficit, you should take the detour from Lires, out to the beach at Lires and then you continue a little further along the coast. It is a short loop off the "main camino", well marked in either direction, and gives you some very nice coastal walking, if only for a km.
Buen camino, I know some people on the forum have given a rather "blah" review to this walk, but I personally always find it to be like a very sweet and special dessert after my Camino.