The Pilgrim Office counts starting places on Caminos INTO Santiago. I do not know (or recall) if it is possible to obtain a Compostela if you start from either place and walk to Santiago. In my recollection, the distance is less than 100 Km, even if you start at Muxia and swing by Finisterre, then head east, towards Santiago.
However, humans being clever and industrious, I suppose there is some way to start at either place and walk in an indirect route to Santiago that may use a section of another Camino route, and lead you into Santiago after 100 + km.
If anyone out there knows this, please jump in. I would have to be at the office to sort this out entirely.