Thanks for the responses.
I have thought a little bit about it and would like to share that with you:
1) We'll spend one full day in Porto to enjoy it - also to recover from the flight etc. Therefore, we'll actually start leaving Porto on the Tuesday.
2) However, I think we'll rather do a "transfer day" in between the route than stressing too much. Therefore, my "planned" schedule looks as follows:
Tuesday, 21.03.2017 - Porto -> Vila do Conde (30km)
Wednesday, 22.03.2017 - VdC -> Barcelos (30km)
Thursday, 23.03.2017 - Barcelos -> Vitorino dos Piaes (18 km)
Friday, 24.03.2017 - VdP - Valencia do Minho ( 54km, Bus, Taxi?), walk to Porinno (20km)
Saturday, 25.03.2017 - Porrino -> Redondela (20km)
Sunday, 26.03.2016 - Redondela -> Pontevedra (20km)
Monday, 27.03.2016 - Pontevedra -> Caldas de Reis (20km)
Tuesday, 28.03.2016 - Caldas de Reis -> Patron (20km)
Wednesday, 29.03.2016 - Padron -> Santiago (24km)
Thursday: EXTRA DAY
Friday, 31.03.2017 - Santiago -> Barcelona
With above, we a) avoid the mountain, b) save 2,5 days with the transfer (what do you recommend?) and c) still have one extra day (stay somewhere for another day to recover, be slower, whatever).
What do you think of above? Is this good with the stroller/cart and possible?
Regarding the alternative routes - how do you exactly mean? If it is bad weather then (too much mud) then how do we deal with it?