The whole idea of having a zivug is kabbalistic and we don’t understand what it means and we shouldn’t be basing our lives on it. From what I’ve been taught, it doesn’t mean what everyone thinks it means.
What we should realize is that everything that happens is bashert. If you marry someone, you were obviously meant to marry them, whether they are technically your “zivug” or not. Like everything else that happens in this world.
DY – I was also wondering about that (the developmental disabilities). Maybe the answer is that when Mashiach comes they will be cured and be able to get married, so they have to have a potential zivug since Moshiach can come any day.