Up until mid September, it will probably still be considered high season for prices. Definitely October will be mid season. My sister and I walked the CP coastal from Porto last September and I did not consider it crowded aside from Porto.
As jungleboy mentioned, take the metro from Lisbon airport to Oriente station. You can then catch either the high speed or Intercidades train to Porto Campanhã station. At Campanhã station you will need to change platforms for the train to São Bento station which run very frequently and take...
I too am waiting to book a ticket in October from Santiago to Porto. I recently contacted Alsa and they confirmed that tickets would be available one month in advance.
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