Meikleour,
Your second last post has nothing to do with the last one.
The one I referred to contains two wrong statements:
an air gap opens up therefore the wing antice hot air is lost
No hot air is lost whatsoever, same amount of hot air goes through outboard slats, thermal exchange is altered at most because of an increased heat exchange surface ("the gap").
system is not effective and may therefore be switched off
That is the shocking part. Since when do you switch off a system because it is less effective?
Believing that WAI is not effective at all with slats extended is incorrect. WAI has its limitation, that's it.
I can only agree with the first part of your last post (first 3 lines).
Why do you think that is?
IMHO, it is an aerodynamic consideration here: when slats are extended, wing curvature/AoA are increased, ice accretion is made easier because it's function of the wing curvature. I think this is why we try to avoid such a situation.
Having said that, it is not because WAI has not been designed(/certified?) to cope with extended flight in icing conditions w/ slats out that I would switch it off as you kindly suggested!
Anyway, Wizzaird answered the OP.