Hi Lambert58,
-I'm also arriving to BIO 17/6 at 13.30.
I'll avoid sleeping in Bilbao the first day and take Guggenheim as dessert on my way back because I truly believe I'll-need-a-space between El Camino experience and my way back Home ;-)
I'll leave BIO airport by bus/metro to Las Arenas-Areete and start my Del Norte Walk at the cross of the Viscaya Bridge to head to Pobeña for the night. I'm not sure if the bus passes or not through Artxandra tunnels as u asked because I'm not sure if the ride is direct from airport-Las Arenas or maybe passing Bilbao downtown first.
I've consulted
http://www.enterat.com/servicios/autobuses-bizkaibus-horarios.php
My interpretation is that the one for Peregrinos (as stated) is #3321.
PLS, take this info with a nip of salt because I've never been there and can't assure any of this 100%.
My personal choice is to TRY to let-go and let The Way develop its' way-! (a bit scary for me but... it is what it is)
I see you. Buen camino!