Hi, tam,
If you want to take a train from Santiago to Sevilla (there are no trains in Finisterre), you will have to take a train to Madrid and from there transfer to the Atocha train station where you can get a train to Sevilla. There are fast and slow trains to Sevilla. The fastest, the AVE, takes about 2 1/2 hours from Madrid to Sevilla. An AVE line for Galicia is currently being constructed (and you will have the pleasure of being re-routed on the Vdlp to avoid construction), so I think the fastest train from Santiago is called the Talgo, leaving Santiago in the afternoon and arriving Madrid around 9 pm. That's definitely the most direct way to go by train.
The route south through Portugal will be a bit more complicated, requiring you to take a train to Vigo, transfer there to get a train to Porto, and transfer again to get to Lisbon. From Lisbon, you can go south on the train to Faro, at which point I don't know whether it's train or bus. I'm sure you can get from Lisbon to Seville, but the most direct way is a bus from Lisbon that goes through Huelva. And it takes all day and is very boring (I've done it), leaving Lisbon around 9 and arriving in Sevilla around 7 pm. Even with the change in time zone, that makes for a very long dreary day.
I have also taken the train from Porto to Lisbon and it's not on the coast, so you won't get your "scenic route" anyway. I'm not sure this answers all your questions, but it's a start. Laurie