Don’t worry about it, it’s not a wet room. You’ve got a one off exposure to moisture from the addy & then only to the screw head; deck screws are good enough.
Screw down every 150mm. Don’t forget to seal the back & edges of the WBP & support the board edges down the centre line of a joist; use noggins for any cross board joints but they are best avoided. You don’t need to worry too much about unsupported edges around the perimeter of the room unless they are in a traffic/load area or at the door threshold.
That’s all I do, immediately before I tile the whole area; work the addy well into the tape mesh & avoid tile grout lines coinciding with board joints as far as is possible. You can get “special” & expensive Knauf tape & sealer & some use silicone but you need to be careful not to spread it around too much as not much sticks to silicone including tile adhesive.
Just close the bag up tight & keep it somewhere dry, it won’t be a problem; shelf life is around 12 months unopened but it won’t “go off” like plaster.
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