1. Wing anti-ice does not work with slats exrended.
It does on the B737.
yotty Wing anti-ice may operate but it wouldn't be effective.
It is also effective.
Why should the extended flight with slats extended,in icing conditions be avoided?
Because you may ice up mechanical areas inside the flap which may need maintenance to inspect after landing,hence the requirement to maintain flaps 15 or more after landing until shutdown.
Why should we avoid remaining in a holding for a long time in icing conditions?
Because eventhough the aircraft you operate is certified in icing condition,it is not in severe icing and more than 40 minutes in moderate i believe.
The protection on maneuvring(vref) is demontrated with icing accumulated on non deiced area of your aircraft(stabiliser,some slats..)up to 3 a certain limit,i believe around 2-3 inches.(40 minutes in mod icing).
If you are staying in icing longer than that you are on your own....