Home again and now I have access to all my Camino books and notes.
Gerald Kelley's 2015 guide to walking the Camino del Baztan was useful also NualaOC's. notes to be found on the Baztan sub forum.
Breaking up that first day with a stay at Ustaritz sounds good. Hot and exhausted, we stopped there for lunch, sat for too long then found it a hot, long slog to Espelette.
Ustaritz signage not always very obvious to
Urdax , stay at the monastery there, check in at the tourist office first.
Urdax to Elizondo would be doable though we were tired and glad to stop at Arizkun at one of the guest houses owned by the lady who runs the nearby bar / shop.
Berroeta, a very good hostel but it is in a village with no shop or restaurant, as others have warned, you need to take food for that stop over.
After the climb up venta san blas we were glad to stop at Lanz. There's an Albergue there plus across the rd. a casa rural where we 4 were pleased to stay. 25€ each we had 2 twin bedded rooms, all facilities and the whole house to ourselves. The lady in the shop manages the Albergue , shop and Casa rural!
Our last day from Lanz to Pamplona was too long. Ideally it would have been better to take an extra day and stay overnight in Sorauren.
How difficult is it?
That will depend on the weather, your age/stamina and walking ability .
No facilty for transport of rucksacks as far as I know.
We 4 did it and survived the late September heat and some rain. Our ages, 71, 71, 64, and our 36 yr old youngster!