That's the problem isn't it? You're asking for certainties, and politicians would love to give you certainty, but in all honesty they can't, because it's far too complex to offer certainty. They might be able to say if we leave that there's a probability xyz would happen, which would be honest but unlikely to yield many votes.
Likewise, the stay group might say that if we stay there's a chance we can reform the EU, or there's a chance that Europe may remain united, but that's all it is, a probability.
It would serve politics much better if they started using 'might' rather than 'will', but I doubt that will ever happen.