Petet,
From my own research I believe that trainee flight engineers did no flying until they reached OTU and crewed up - ie after they had graduated from St Athan and received their brevets. Cliff mentioned something about this on this thread a year or so agao. I also asked another former flight engineer - Tom Knox of 149 and 199 Sqns on Stirlings - who wrote me a letter last year that confirmed this.
I don't have a full syllabus but from inputs from both Cliff and Tom I cobbled together a short description of flight engineer training on my blog - you can
find it here.
I also believe the white flash denoted an airman under training. One of the veteran aircrew I talk to regularly tells me they used to take it off when they wanted to abscond temporarily from their Initial Training School by diving through a hole in the fence - this way they would look like permanent staff rather than recruits while walking innocently down the road to the train station!
Adam