Thank you Neil for your nice feedback.
yes, I use a Salota passive 10 slot backplane I got on eBay for 90 Eur (the connectors were already soldered). I helped me get started but I am awaiting the 18-slot active backplane from Ants. the Salota board grounds pins 61 and 70 for some reason and I could not cut the tracks as they are located beneath the connectors..
as for the IDC connectors you are right : I had to swap every 2 wires on the ribbon cable (ie 1 and 2, 2 and 3 etc) otherwise the two rows got swapped. This comes from the fact that I wanted to mount the mezzanine connector from the copper side. Swapping wires on the ribbon appeared easier than I anticipated : separate each 2-wires, swap them and fix them on an adhesive tape to keep them in place while you insert your connector and mount it. If you do not want to swap the wires, you can solder a 90 angle connector on the component side and use a regular cable. It will simply add an extra space between the board and your panel.