Sorry, this is looonngg.
I have biked different parts of the coastal route covering the section between Porto and Vila Praia de Âncora. So its to this section my advice applies.
If you mean mountain biking, except for short sections, the terrain is always is ridable. To me, these were never an annoyance I'd want to avoid, I'd rather stick to the route.
I do recommend using a backpack rather than paniers. It makes the bike more cumbersome to maneuver on and off the bike.
If you're riding a road bike, many sections become harder or impossible to cycle. Even many road sections, which being roughly paved can really hammer your arms, backside and bike.
As to the route itself, there are indeed conflicting indications, even in the Via Veteris site.
There is waymarking through the city of Porto, but I recommend leaving along the river to the sea. Keep close to the sea and go north. Trafic can be heavy but generally slow since is kind of a promenade route. There are frequent broad sidewalks you can use as well. Near the refinery north of Matosinhos, the boardwalks start, and you can go on those for most of the way up to Vila do Conde. On the bits of road you may use traffic isn't too bad since their secondary roads. There is also, a bit of cycle lane in Perafita and Lavra. There is a bit near Labruje with a few minutes detour in-land and some stairs, these are the only obstacles.
There are waymarking that take you paralel to the airport (between the central route and the sea). From what I've seen, its minor paved roads.
In Vila do Conde, I suggest turning left at the bridge and following the river to the sea, beautiful part of town, and when you hit the sea, there is a cycle lane next to the fort which takes you up to the north end of Póvoa do Varzim. I've seen gps files which take you through the middle of these towns, I don't recommend it, too many intercessions, even if traffic isn't heavy, its less safe and a nuisance.
After the cycle lane in Póvoa, keep hugging the shore and there are more quite roads and boardwalks paralel to the sea. These take you up to Aguçadoura where the waymarking pops-up again.
Following the arrows you enter Apúlia. The camino here is mostly on compacted sand, very good to ride on. Up to now MTB or road bike with skynier tyres (1.5 or similar) would both cope well, but now the terrain will become more challenging.
You can continuing hugging the coast, but it takes through more paved roads than the waymarked camino. North of the Cávado river (you cross it in Fão) the arrows take you along paved roads, not badly paved, but still not the best. As it nears the Neiva river, the camino goes along a lovely single track paralel to it, you'll have to dismount here.
After crossing the river, on the waymarked path to Viana do Castelo, there is a forest with some sections which might require walking, but most of it is small roads, some with bone shattering pavé.
An alternative here: I have seen pilgrims on foot and on heavily laden bikes taking the EN13-3 road instead of the following the arrows to Viana. There is less climbing and its a tarmacked surface, but traffic can be quite fast and the road shoulder small on non-existent.
In Viana, the Camino takes you through the town and exits close to the hills. Its good, a bit less so as you exit the town. Exiting the town and just north in Areosa almost all its small roads tarmacked and paved. After this in Carreço it crosses the rail road on a level crossing, be careful.
Alternatively you can follow the river after crossing into Viana, also very lovely views of the city. Go around, north of the shipyards, take the N13 and after a roundabout you'll find a walking/cycling lane paralel to it all the way to Carreço.
The waymarked path takes into the hills in Carreço and Afife, with more all-terrain riding. Some beautiful bits, but can be challenging. Some walking may be required. Only time I lost sight of the arrows was on a forest road going down towards Vila Praia de Ancôra (too distracted enjoying the descent).
The far easier option is to take the N13 northwards, its straightforward. It is the main thoroughfare on the coast, and especially in the Summer has a lot of traffic, but there is a good shoulder on the road, so I'd say its fairly safe.
If you have any queries about this section of the Camino, I'd be happy to try and answer.
