[JonnyA]

hI
can't offer any technical advice here but interesting reading...
obviously you haven't posted the whole report but a couple of things spring to mind that should be asked of this based on the posts:
Is the report independant, who commissioned & paid for it, is the 'expert' known the dealer & what are their proffessional qualifications?
Just because a part is generally reliable does not mean that they never fail. Does the report mention testing the oil level sensor and associated systems? if not the report is at fault since this is the only way for the owner to check the oil and without knowing this they cannot make reliable conclusions on whether the oil was checked or not.
If they did not witness the draining of the sump they cannot report it as fact that there was only a certain amount of oil removed (the garage are an interested party).
Does the report include all of the fault logging within the vehicle? if not a big hole in their investigation since modern cars are built around the computer systems.
I understand that the volvo system logs fault codes with a time line (so my local indy tells me) so the codes you can see should give you a story line.
The report should record all of the observations of the inspecting person, the likely causes of any damage and the mechanism or processes that may have lead to this. From the analysis of this evidence they can draw conclusions, which may draw on their experience, as to the likely source(s) of the problem.
I would say that they could only postulate that 'lack of maintenance' by the owner was the cause when they have eliminated all of the possible 'internal' causes.
if you have been checking the oil and no warnings came up then i can't see its anything other than a vehicle fault ... just a bugger arguing the case.

i hope that you get this sorted out sooner rather than later.