You don't at all. This is not a batman gambit.
You do, actually. This isn't about Loghain simply not having accurate information. Everything he says on the subject of Alistair's birth indicates his belief that Alistair was conceived and born during Rowan's lifetime, and that he was hidden on that basis alone, to keep Rowan from being humiliated and reduced to a concubine in the eyes of the people.
The only possible explanation for that would be to say not that Loghain simply had inaccurate information, but that he was an outright liar. But that alone wouldn't work because someone, somewhere, would be able to add up the dates of what happened when and figure out that something didn't jive.
The entire story of Origins reads exactly as if Alistair was intended to be the product of Maric's indiscretion. If he turns out to actually be Fiona's son, so be it, but there's no point at all in pretending this was the intention all along when there's a mountain of evidence that proves it to the contrary.