I understand that the introduction of diesel can reduce the amount of blow by gasses and thus reduce the return of oil to the reservoir.
That's correct. The 912,
AFAIK, uses a mechanism whereby the blow-by from the cylinders causes enough pressure to develop in the crankcase to push the oil into the separate oil reservoir.
If the diesel fuel has somehow impeded this blow-by (soot on the cylinder walls maybe?), you might actually have been running the engine "dry", with all the oil collecting in the crankcase and not making its way (via the external tank and oil pump) towards all the bits and pieces that need lubrication. That would explain the dropping oil pressure and rising oil temperature: a lack of circulating oil.
The resulting wear on all the surfaces might then be the reason for your increased oil consumption afterwards, and for the debris in the filter.
As others have said, contact the importer (Skydrive I think) and have them do a detailed inspection.