Both of those theories would be interesting if played out.Either Kristen switched those babies first, or saw it being done. (or the mag article has taken liberties) But if Kristen knows, she maybe thought the grieving would cement Brady to her more than a child (since he already has one with Theresa, & she is in CA with Brady's son, so it did not bind them together forever)
And desperate Kristen, in order to get Brady into bed, came up with the Nicole mask, as Nicole pointed out to her once.