It's not going to be open in april. That'd be a really stupid decision as there are heavily discounted tickets available throughout april.
I agree and I disagree. The discounted tickets are a way of boosting sales, but are also a way of trying to keep the public satisfied after removing the pay once, visit twice scheme. Of course, the opening of a new area boost sales naturally, however the opening date of Rookburgh hasn't been announced yet. Notice that the discounted tickets are only for a limited time only, as advertised on their social media. Therefore, I think it would be perfectly possible and feasible to see the discounted tickets removed in a couple of weeks time, and then be followed up by an announcement of Rookburgh.
That way, you have boosted ticket sales during the off season, and then you will have a second surge of ticket sales when Rookburgh is announced, with people paying full price.
I agree that April is unlikely, but I'm not going to rule it out completely, as construction is really progressing now.
As for the new screens, if Rookburgh is to open in say June, then it's perfectly easy to take F.L.Y off the new screens until it opens up, so as said above it's just for test purposes and doesn't really give any information as to when it's going to open.