That's really interesting. I am making something of a study of the RFC at the battle of Messines. I'll drop you an email - we should swap notes.
Here's what Passingham has to say on German over-confidence:
"The Commander of Artillery, Gruppe Wytschaete, was convinced that his guns would easily defeat the British in any artillery duel, although the evidence for such optimism was thin." - p.83
Passingham notes that the Germans discovered the Petit Douve Farm mine (by tunnelling) and made successful trench raids on the mine shafts at Hill 60, Trench 122 and Factory Farm. However, he writes:
"In April, he [von Fuesslein, Kommandeur der Mineure] had reported at the fateful meeting of field commanders and Crown Prince Rupprecht that his counter-mining had been so successful that a major British mining attack against the Messines Ridge was no longer possible. ... It was decided that ... a major mining attack was unlikely." - p.86
It seems like a well researched book so I'm inclined to accept this analysis.