hi.
1. I'd advise to start in Barcelos. You can take a train from Porto.
Barcelos - get to the Bom Jesus da Cruz Church in the center of town for the first stamp on your camino.
Vitorino dos Piães - arrive at
Casa da Fernanda, call ahead to make reservation.
Rubiães - it is a long day over the Labruja hill
Tui - short day to rest, more time to see Valença fortification and Tui old town.
Redondela - another long day, avoid the industrial area in Porrino.
Pontevedra - short day to recover, more time to see Pontevedra which is a big city.
Caldas de Rei - not a long day, but you can rest your feet in the thermal fountain in town.
Padron (or Herbon) - try the pimentos and go to the church near the albergue for a good view.
Santiago
To enjoy talking with other walkers, everyone says Casa de Fernanda is one not to miss. Starting in Ponte de Lima means having the Labruja hill right in the first day, it might be a shock to the system...
3. Its vacation month, so more pilgrims on the camino.
4. There are some private albergues, these will be more expensive, but still less than hostals, pensiones or hotels. You can ask in the albergue what alternatives they suggest. Sometimes you can walk 5km more and find another public albergue. I've arrive in Porrino to find the albergue full, asked about the following albergue, they called, also full. They let me stay on the floor.
5. What is the temp rating on the bag. Rooms full of pilgrims tend to get very warm on the summer. A down bag might be overkill, some people use just a bag liner. I've used a 15ºC light and compact bag and was very happy with it. Cheap and with a full zip you can almost make it as a simple cover to reduce warmth.
All the best, any doubts we are here.