James
A brilliant post which will get you all the assistance you deserve from all of those with experience of this system. Tony's advice is spot on, the injectors on that system fail if you cough near them, the HP pump is a common fail item, failing to generate sufficient pressure and pressure decay excessive, Vapour in the leak off lines??, could be combustion pressure leaking back through injector(s)or air entrained in the supply side?.
I would be strongly inclined to replace egr valve and advise the customer accordingly---unless, like myself and others on this forum, you want answers to the problems presented to you, very admirable I know, but it's taken me many years to realise that bills and wages are not paid by that type of work ( I always thought I would make it up on the next one), bet that sounds familiar to some!.

Ed Kaps