more variants:
Ours (-4/500) close the WAI valve when thrust levers are moved through the TKOF warning position (lights go bright blue for switch/valve disagreement). At lift-off, only the switch trips to (correct) OFF position.
Precisely as described in VOL2 pg. 3.20.6.
This is also performance wise should engine fail between V1 and Vr. In case you need WAI on ground, it is most likely that the runway will be at least wet so there will be some time to go from V1 to lift-off. Our engines are not restricted to failing when airborne only.
Can be reselected any time when the AC is in air mode (click at liftoff), i.e. immediately. However should I really NEED the WAI after take-off, I'd decide not to fly at all.
Cheers,
FD