GinFizz

Thanks - unfortunately for the ICE/TGV trains I need to take the DB booking system is only showing pricing options for either non-flex (Super Sparpreis) or fully flex, with "semi-flex" (Sparpreis) greyed-out as unavailable. Good to know for future travel though.

So in the end I ended up booking exactly what I said I wouldn't book in post #3 - namely a ticket from Koeln to Lille-Europe, with the plan to buy a ticket for Duesseldorf airport to Koeln separately on the day using the DB app. It turned out that by the time I booked this was only a few Euros extra than a "Super Sparpreis" trip starting from Duesseldorf (requiring, in theory at least the extra 2.70 for a Sky train ticket).

Thanks to this thread and the Google searches it has inspired, I now understand what a "VRS price level 5” ticket is, and why you need to buy a VRS ticket even though the journey starts inside the VRR region… I have also learned that GoogleMaps for timetables can require care - you need to actually type "Duesseldorf Airport Station" for it to pick up any trains leaving from the non-terminal station (otherwise it only shows routes that connect through Duesseldorf city, and doesn't list any routes using the main airport station).

Many thanks again to all above for the help and advice - I shall raise a toast to you all as I enjoy a beer + wurst lunch in Koeln before my train to France!