I walked the Central route north of Porto, so my thoughts are based on reviewing
Brierley's guide and the OSM mapping of the area north of Porto.
It appears to me that the first day of the Senda Litoral, on the day walking out of Porto, is the only day where there aren't good options to mix and match that and the Caminho da Costa. That said, there are sections where it appears that you would be on whichever you have chosen for a few hours before the two routes come back together.
As for options to cross to the Central route before Redondela,
Brierley appears to really only give two. The first is to head inland to Sao Pedro de Rates from Vila do Conde, and continuing onto Valenca from Vila Nova de Cerveira. These make some sense as the places where that transition is likely to be shorter than doing it elsewhere.
Other options would be to head inland to Barcelos or Ponte de Lima. Both are on rivers, and linked to the coast by major roads, but whether there are suitable walking tracks beside either the rivers or roads isn't clear.
Of course, walking all the way from Porto to Redondela on the Caminho da Costa without doing much of the Senda Litoral beyond Vila do Conde would also be an option. I have considered that as an option for a future pilgrimage.
Edit:
Brierley recommends
http://www.caminador.es/. I haven't explored it in any great detail, so while it does appear to offer some other options, it doesn't show routes across from the coast to Barcelos and Ponte de Lima. I am prepared to suggest this means that they aren't really good options, but that assessment might be given the general credibility of 'guess'!
