A few answers for a few of your questions:
1. Bikes usually follow a different route than the walking path of a Camino. The Variant does follow a lot of roadways, though, so you’d be basically next tot he walking path but I’m not sure you want your 7yo sharing the road with Portuguese drivers (trust me, they are definitely some of the worst!). Plus you’d miss the absolute best part of the route: the “Path of Stone and Water” which follows next to a stream.
2. The boat MAY be able to take bikes, but you won’t know until that day since they have several different boats depending upon how many passengers are going and the tide schedule. Generally, they take either a small or medium boat, neither of which would not be able to fit your bikes.
3. Any motorized vehicle (including eBikes) disqualify you from a Compostela.
4. No idea about trains.
5. Paradors are generally larger luxury hotels and will have room for bikes, but albergues will be completely hit or miss.