Sorry for the delay in picking this up again - I'm trying to move house and everything is a mess.
But I have found time to recheck VB on Soult's intentions on 29/7/1813. He says that Gazan's orders suggest that S intended to complete the first stage of an urgent march on the road from San Sebastian to Sanesteban so that by 30/7 the whole of his army would be around Lizaso, but that S would give further orders when he reached Lanz (named as HQ for the day) on 30/7.
Drouet D'Erlon's orders are given as "se rendre a Ostiz aux environs de Lizaso avec ses 3 divisions d'infanterie et la division de dragons de Treilhard, tenir tres fortement les chemins de Pamplona et pousser des reconnaissances au col de Dona Maria et vers Irurzun".
Not sure if this means to cover the Ostiz-Lizaso area or to move from Ostiz towards Lizaso - if the latter, which chemins de Pamplona was he to hold, and who was going to secure the road at Ostiz that the rest of the army was hoping to take? But it does suggest that S recognised he needed more info about the area W of Lizaso before he committed the whole of his force in that direction.
Bear in mind also that his army was v. short of supplies and he had sent orders to Bayonne on 28/7 for all supplies that could be packed on mules to be sent to Lanz via Elisondo by 2/8 - this suggests to me that a movement further west* was still only an option when there was a better chance of getting to supplies quickly by continuing north from Lanz on the PdeV road.
(* He had also ordered supplies west, to Villatte's most forward position, based on the belief that V would have crossed the Bidassoa and be on his way towards San Sebastian, so he might have reckoned that his people would get access to those supplies if he could get them across to Hernani or Tolosa. That is his explanation in his subsequent account of these events.)
According to VB, having received his orders for 30/7, DDE took the precaution before daylight (that is to say, probably soon after Soult left Zabaldica to cross the field of Sorauren) of putting Darmagnac's division on the heights of the right bank of the Ulzama, facing Hill's Portuguese - they had been in contact with these troops before the movement started. Abbe then moved up to Lizaso, followed by Maransin. They found Hill across the road to Irurzun with his left on Beunza, and Abbe then took on the Allied left while Maransin tackled the centre.
Whatever he claimed afterwards about meeting DDE on the road and urging him to action based on the deserters accounts of Wellington's plans, it seems more likely to me that the fighting around Lizaso was more DDE's response to opportunity than Soult's grand design, and that when S left Zabaldica his prime objectives were lunch and a siesta at Lanz while two thirds of his army (all of whom - Soult included - had entered Spain via Roncesvalles, so didn't know the ground) came up from the south to join him, and the rest found out whether there was a western alternative to the road north over the Puerto de Velate.