Damn, Google AGAIN sent the notification of Mike's reply to the server's Spam folder despite me previously marking it as NOT spam (I don't normally use web-mail, so rarely check that folder). I've now updated my ddisoftware.com profile to change to a non-Google eddress that I have some control over. So, sorry for this delayed response.
Mike,
Today I had a call from an Epson support technician, who explained to me why the R2880 driver (and I'm assuming all their other inkjets with borderless capability) reports the incorrect paper size. So I came here today to explain it for everyone else's benefit, only to find your far more detailed and perfectly correct explanation!
Many thanks for taking the time to explain this behaviour, and I'm sure others will benefit too from this understanding. I like your suggestion of setting 'compensating' margins in Qimage, which would solve my edge-positioning problem for multiple images. I accept that I might get a sliver of white on an edge due to paper-feed misalignment (the Epson tech also explained that), but I only occasionally use this setting when I want the pictures at the maximum size possible for the paper, without losing a few millimetres from the edges due to unwanted expansion. Trimming off the little edge sliver is minimal area loss compared to what would be lost using expansion.
Thanks again for your excellent and patient support.
Daniel