Together with my wife I walked the caminho da Costa in May 2015.
From Viana do Castelo so not the f ull start from Porto. We did that before in 2013 .
The Coastal is quiet but I expect more pilgrims walking in May that route .
When we walked there was only a handfull .
I should not advise a woman alone to walk from A Guarda to Mougas. You walk around a mountain far from any urban area. Not that anything happened but...in case .
If I were you I should walk with others for that specific part, about 5 kms of length.
You know before who is going that way long before and you always can decide to follow the central route if you doubt, even in Caminha where instead of taking the ferry to a Guarda walk to Valença and follow the central route from there.
Other option is take public transport or a taxi from a Guarda to Oia where is a hotel or Mougas where is an albergue.
The coastal is safe but special that part you could feel very lonesome.
See an impression of that part. after the crossing of the river Minho is a Guarda.
Very nice caminho by the way !
Bom caminho