Yes you can drive with the boot open. I took a couch to the tip. Obviously you can't use the power tailgate, put you can manually put it down till it rests on whatever is sticking out.
I just used a bungy from the rear wiper and clipped it over the plastic bumper.
Stuck to A roads but it's about 6 miles to the tip for me and no issues. Can't remember it bonging while driving, obviously the dash just showed the boot open.
I didn't check it in Xc40, but my in previous XC60 is was possible to bend the front passanger seat forward.
That gave me lost of space to stransport long objects.
with the rear seats folded and the wood beams coming between the driver and passenger seats I managed to get 2.4m lengths in and shut the boot. Useful!