Rain It's not knock back, that would give a low pedal, but no delay. And it wouldn't disappear in dry weather.
It is in fact wet discs for sure. The conditions are quite well defined and I have a good friend with the same experience on a 320d M-Sport. I have also found posts on the web of people describing exactly the same conditions and symptoms. Could be the way the M-Sport bumpers channel cooling air for the brakes. My previous Sport (no M) had different ducting and wasn't that bad in the rain. (It also had different rims)
Do consider that factory pads have a long and very shallow chamfer. This forms a wedge with the disc that can hold a lot of water.
Cracks We have to see what happens. In favour of low risk is the street use (even if I do enjoy a winding road, it's never as hard on brakes as a trackday.)
The Zimmermann discs do seem to have evolved to reduce crack formation and propagation if/when they form. The holes aren't that large, chamfered* and none are radially aligned.
My choice of ceramic pads might be problematic, if indeed ceramic pads aren't good at conducting heat away from the pad/disc interface. Still, this is hard to verify because there are so many rumours and myths on the internet.
*Yes Ovekam, as you say, once worn away the straight edged holes will crack far more easily. However, the disc starts life 22mm thick and minimum is 20.4mm. Once the chamfer is worn away, the disc is strictly speaking ready for replacing anyway.
What pads were your friends running? What type of driving?