The only baby switch that would make sense is Rex since she slept with Rex and Eric within the same time period. So those are the only 2 candidates.
If a Xander had a part of a baby switch for Mackenzie, that would actually drive Sarah and Xander apart for right now.... that would mean that Xander (and possibly Victor) allowed her to get attached to a child that was never hers and in the end she'd have to give the child up anyway... she would see Xander as a monster (at least for a little bit) for that...

But I do like your story... that seems like something that could've happened.