Michate, the information about division transfers came from the relevant volume of the 'Der Weltkrieg' series. It is very rare. I have the Reichsarchiv volume 27 in the Schlachten des Weltkrieges series, 'Flandern 1917'. It gives an interesting overview but is not very detailed. It does mention the 12th Division in reserve as an Eingreifdivision (counter-attack division) behind the Stellungsdivisionen (frontline divisions) of die Gruppe Wijtschaete. The 12th Division was not heavily involved on July 31st because the British attack in this sector was a minor 'bite and hold' operation, designed to protect the southern flank of the main attack north of the Ypres-Comine Canal. Jack Sheldon mentions that the 12th Division was subsequently involved in the front line, after it relieved one of the divisions that was worn out.
Very interesting, though I was thinking at the Volume 12 of the "Der Weltkrieg" series, not the "Schlachten des Weltkriegs" series, though both were created by or at least under the auspices of the Reichsarchiv. Buts as mentioned, I have no access to that volume, unfortunately (though some time ago I went to a similar list attached to one of the volumes on the Somme battle).
The main effort of the British attack was along the Gheluvelt ridge. The German defence did extremely well here. Although the British forces broke through the first line very easily (as it was not heavily defended in keeping with the defensive tactics), the operations on the ridge broke down as a result of the unsuppressed newly-built concrete MG pillboxes in the wooded areas. The high ground was captured, including Stirling Castle, but the British did not achieve their second line of objectives. This exposed the attack around St Julien. Initially, the British attack was very successful here. The German defenders on the ridge, who had not been reached by the attack on the ridge, were able to enfilade the British attackers lower down. This made it much easier for the Gegenstoß (immediate counter-attack) to suceed in this sector. Further north, the British retained their objectives and beat off the counter-attacks.
According to Wynne, German preparedness to risk counterthrusts and counterattacks was clearly based on value of terrain. This was seen as much more expendable in the northern and central sectors (front of group Passchendaele), while possession of ground was much more critical in the area of group Wytschaete. Actually most commanders had been prepared to retreat to the Flanders (I) line (approximately in the vicinity of Passchendaele) after the Messines calamity.
The decision to accept battle in the current lines then seems to have been taken by von Lossberg.
The Zabecki material is very clear. His book is very good, though I don't agree with some of his conclusions, FWIIW. It is the best study of the German spring offensives in English, by far.
It is one of the most interesting of the book, and IMHO the whole topic of logistics merits much, much more, serious study. German logistics, troop movements and railway operations in particular seem to be virtually unexplored (I think for the British, at least one or two good studies exist), though they were so crucial to the course of events (more so than the specifics of a handful of Sturmbattalione IMHO). So you are left to the bits and pieces intermingled to the official histories.
I would be interested in your points of disagreement with Zabecki and, though his study is impressive, I am myelf sceptic about one or two of his views, but that would be stuff for another thread.