I'll respond to my own questions. Picked up the car last weekend. It has 5 fixpoints on each side. One is way forward and the other 4 are evenly spaced from the rear with about 60 cm spacing (maybe more, did not measure).
I bought an extra set of Fiamma awning brackets for the Ducato fixpoints which I modifyed by cutting off the actual awning mounts. Then I installed an aluminium "L" profile to this bracket. And finally I installed the Thule XT751 foot to this L profile. The result is that the Thule foot is shifted about 5 cm inwards from the original fix point location and now it clears the awning with a couple of cm.
The roof bars are not sharing fixpoints with the awning. But it would be possible to make a similar solution for shared fix points as well.
So now I can go windsurfing and still be able to cool down in the shade