Well since the locals will still be on their holidays, swimming and surfing along that coast, then you should be fine
More likely to be too warm than too cold so the sleeping bag liner should be fine certainly for the beginning of September -...
I too am somewhat mystified at your seeming re-invention of the wheel: it's not an orienteering contest but a quiet road / trail based walk through nothern Spain
Of the books, the best maps are in the Wise Pilgrim editions - there is one for the...
Pretty much impossible unless you take a taxi from Santiago Airport to Sarria
From SCG would need to take bus via Lugo or train via Ourense and at least 3 hours (and last bus to Lugo leaves at 8pm)
From MAD bus via Lugo would be at least 8 hours