By bike & alone, is it safe?

All reports are that it is safe. Have a good trip.

Por mi pueblo, Soto del Barco, todos los días pasa algún grupo en bicicleta haciendo el camino.

I would be absolutley confident about being safe, no worries there.

Yes, you will meet other cyclists, but not many.

Although we cycled all the way from home (UK) to Guernica (in stages), we walked from there to San Vicente (almost at the end of Cantabria). I would recommend that you get a good road map (as large a scale as you can afford - we had a michelin map, and cut off the coastal strip which was all we needed. If you combine that with any guide you have and leaflets you pick up from Tourist information (which are very useful for albergues etc) you should be able to create a route which uses yellow arrowed routes (there seem to be some alternatives on the Norte) while sometimes having to follow small roads when the path is not possible.

I am really jealous - wish I could come and cycle with you! We still have to complete our camino from San Vicente so hopefully you will be able to tell us about the route from there.